34.1. A Coelom (p. 594)
A. Protostome Organization
1. Protostomes are bilaterally symmetrical, have three germ layers, the organ level of organization, the tube-within-a-tube body plan, and a true coelom.
2. The coelom is a body cavity between the digestive tract and body wall that is completely lined by mesoderm.
3. Coelom allows digestive system and body wall to move independently; internal organs can be more complex.
4. Coelomic fluid assists respiration and circulation by diffusing nutrients, and excretion by accumulating wastes.
5. This cavity may serve as a storage area for eggs and sperm.
6. Coelomic fluid protects internal organs and also serves as a hydrostatic skeleton.
B. Coelomates Are Protostomes and Deuterostomes. (Fig. 34.1) [transp. 182]
1. Protostomes include mollusks, annelids, and arthropods; they possess three embryological characteristics.
a. In spiral cleavage, there is cell division without an increase in size of cells; the fate of the cells is fixed.
b. The blastopore is associated with the mouth.
c. The coelom (schizocoelom) is formed by a splitting of mesoderm on either side of the primitive gut.
2. Deuterostomes include echinoderms and chordates; they also have embryological characteristics.
(Fig. 34.1)
a. In radial cleavage, the new daughter cells sit on top of previous cells; fate of cells is indeterminate.
b. The blastopore is associated with the anus.
c. The coelom (enterocoelom) is formed by fusion of mesodermal pouches from the primitive gut.
34.2. A Three-Part Body Plan (p. 596)
A. About 110,000 species of mollusks belong to the phylum Mollusca.
1. Mollusks have a three-part body plan: a visceral mass, a mantle, and a foot.
2. Most are marine, but some are freshwater and terrestrial. (Figs. 34.2- 34.5)
3. Visceral mass contains internal organs, including digestive tract, paired kidneys, and reproductive organs.
4. Mantle covering does not completely enclose visceral mass; may secrete a shell, contribute to gills or lungs.
5. The foot is a muscular organ adapted for locomotion, attachment, food capture, or a combination of functions.
6. A radula is an organ that bears many rows of teeth and is used for grazing on food.
7. The nervous system consists of several ganglia connected by nerve cords.
8. In mollusks, the coelom is reduced, limited to the region around the heart.
9. Most mollusks have an open circulatory system: a heart pumps hemolymph through vessels into a hemocoel; blue hemocyanin is the respiratory pigment found in mollusks.
10. Some are slow moving with no head; others are active predators and have a head and sense organs.
11. Chitons are in the class Polyplacophora.
a. Chitons have a shell that consists of eight overlapping plates.
b. They have a ventral muscular foot used for creeping along or clinging to rocks.
c. They scrape algae and other plant food from rocks with a well-developed radula.
B. Bivalves Have a Double Shell
1. The class Bivalvia contains the bivalves (clams, oysters, mussels, and scallops).
2. "Bivalves" are two-part shells that are hinged and closed by powerful muscles. (Figs. 34.2b and 34.3)
3. They have no head, no radula, and little cephalization.
4. Clams use their hatchet-shaped foot for burrowing; mussels use it to produce threads to attach to objects.
5. Scallops both burrow and swim; rapid clapping of the valves releases water in spurts.
6. The bivalve shell is secreted by the mantle.
a. The shell is composed of protein and calcium carbonate with an inner layer of pearl.
b. Pearls form as layers of shell deposited about a foreign particle inserted between mantle and the shell.
7. A compressed muscular foot projects down from shell; by expanding the tip, it pulls the body after it.
8. Beating cilia of the gills causes water to enter the mantle cavity by way of the incurrent siphon and to exit by way of the excurrent siphon. (Fig. 34.3) [transp. 183]
9. While cilia of gills move water through mantle cavity, gills also capture particles in water and move them toward mouth; mouth leads to stomach, which leads to intestine, which passes through heart, ends at anus.
10. The circulatory system is open.
11. The nervous system consists of three pairs of ganglia.
12. Two excretory kidneys below heart remove ammonia waste from the pericardial cavity into mantle cavity.
13. Sexes are separate.
a. The gonad is located around the coils of the intestine.
b. Certain clams and annelids have the same type of larva, indicating a possible evolutionary relationship.
C. Cephalopods Have Heads
1. The class Cephalopoda ("head-footed") includes squids, cuttlefish, octopuses, and nautiluses.
2. Both squids and octopuses can squeeze water out of their mantle cavity so that water is forced out through a funnel, propelling them by a form of jet propulsion.
3. Around the head are tentacles with suckers adapted for grasping prey.
4. They possess a head equipped with a powerful beak used to tear prey apart.
5. They have well-developed sense organs, including focusing camera-type eyes.
6. Cephalopods, particularly octopuses, have well-developed brains and show a capacity for learning.
7. Nautiluses are enclosed in shells, but squids have a shell that is reduced and internal; octopuses lack shells.
8. Squids and octopuses possess ink sacs from which they squirt a cloud of ink, as a mean of escaping predators.
9. Squids possess a vestigial skeleton under the mantle, called the pen, which surrounds the visceral mass.
10. Squids can use the mantle cavity to squeeze water out to move either forward or backward.
11. A squid has three hearts, one pumps blood to internal organs; two pump blood to the gills in the mantle cavity.
12. Gonads make up a large portion of the visceral mass; sexes are separate.
a. Spermatophores contain sperm, which male passes to female mantle cavity using a tentacle.
b. After eggs are fertilized, they are attached to the substratum in strings containing up to 100 eggs.
D. Gastropods Undergo Torsion
1. The class Gastropoda includes snails, terrestrial slugs, whelks, conchs, periwinkles, sea hares, and sea slugs.
2. Most are marine but some are freshwater or terrestrial.
3. Many are herbivores that use their radula to scrape food from surfaces; in contrast, carnivorous gastropods use their radula to bore through a surface, such as a bivalve shell, to obtain food.
4. Most have a developed head with eyes and tentacles projecting from a coiled shell that protects visceral mass.
5. Nudibranchs (sea slugs) and terrestrial slugs lack shells.
6. During development, gastropods undergo torsion where body is twisted to bring anus and mantle cavity downward, forward and around to a position above head; this positions visceral mass above the foot.
7. In aquatic gastropods, gills are found in the mantle cavity; in land gastropods, the mantle is richly supplied with blood vessels and functions as a lung when air is moved in and out through respiratory pores.
8. Terrestrial gastropod embryonic development does not go through a swimming larval stage.
9. For terrestrial snails, shell not only offers protection but it also prevents desiccation.
10. The muscular foot contracts in peristaltic waves from anterior to posterior; a lubricating mucus is secreted.
11. Terrestrial gastropods are hermaphroditic.
a. In premating behavior, they meet and shoot calcareous darts into each other's body wall.
b. Each inserts a penis into the vagina for the other to provide sperm for future fertilization of eggs.
c. Eggs are deposited in the soil and development proceeds without formation of a larvae.
12. Hermaphroditism assures that any two animals that meet can mate, especially useful in slow-moving animals.
34.3. Segmentation Evolves (p. 600)
1. Segmentation may have evolved in conjunction with a hydrostatic skeleton.
2. Segmentation is subdivision of the body along its length into repeating units (segments).
3. As animal utilizes a hydrostatic skeleton, partitioning of coelom permits segment independence of movement.
4. Once segmentation appeared, each segment can become specialized to perform a particular function.
A. Annelids Are Segmented Worms
1. About 12,000 species of segmented worms are in the phylum Annelida.
2. Segmentation shows externally by rings that circle the body; internally, septa divide the coelom.
(Fig. 34.7a) [transp. 184]
3. The well-developed, fluid-filled coelom and the tough integument act as a hydrostatic skeleton.
4. The complete digestive system is specialized to include a pharynx, stomach, intestine, and accessory glands.
5. The closed circulatory system has blood vessels running the length of the body and branch to every segment.
6. The nervous system consists of a brain connected to a ventral solid nerve cord; a ganglion is in each segment.
7. Excretory nephridia in each segment collect waste material from coelom and excrete it through the body wall.
B. Marine Worms Have Parapodia
1. Most polychaetes in the class Polychaeta are marine.
2. Most polychaetes possess parapodia and setae.
a. Parapodia are paddlelike appendages used in swimming and serve as respiratory organs.
(Fig. 34.6)
b. Setae are bristles, attached to parapodia, that help anchor polychaetes or help them move.
3. Clam worms such as Nereis are active predators.
4. Many have well-developed cephalization; the head has well-developed jaws, eyes and other sense organs.
5. Sedentary filter feeders possess tentacles with cilia to create water currents, select food particles.
(Fig. 34.6b)
6. Only during breeding do polychaetes have reproductive organs; the zygote develops into a type of larva similar to that in marine clams.
C. Earthworms Hide Underground
1. The class Oligochaeta includes earthworms with few setae, protruding in clusters directly from their body.
2. Earthworms have a poorly developed head or parapodia.
3. Locomotion is accomplished by coordinated movement of the body muscles and assistance of their setae.
a. When longitudinal muscles contract, segments bulge and setae protrude and anchor into the soil.
b. When circular muscles contract, the worm lengthens, setae are withdrawn and the segment moves forward.
4. Earthworms reside in moist soil where a moist body wall allows gas exchange.
5. Earthworms are scavengers that extract organic remains from soil they eat.
6. A muscular pharynx draws food into the mouth; it is stored in a crop and ground up in a muscular gizzard.
7. The dorsal surface of the intestine is expanded into a typhlosole that allows more surface area for digestion.
8. External segments correspond to internal septa, walls separating each body segment.
9. Excretory system involves nephridia.
a. The coiled nephridia tubules in each segment have two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall.
b. Between the two openings, the coiled nephridia tubule allows removal of waste materials from blood vessels.
10. Red blood is moved anteriorly by a dorsal blood vessel and pumped by five pairs of hearts to a ventral vessel.
11. Earthworms are hermaphroditic with testes and seminal vesicles, and ovaries and seminal receptacles.
a. Mating involves lying parallel to each other facing opposite directions and exchanging sperm.
b. Each worm possesses a clitellum that then secretes a mucus, protecting sperm and eggs from drying out.
c. Embryonic development lacks a larval stage.
E. Leeches Are Parasites
1. Leeches belong to the class Hirudinea.
2. Most are freshwater species but a few are marine or terrestrial.
3. Each body ring has several transverse grooves.
4. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker.
5. Although some are free-living predators, most are fluid feeders. (Fig. 34A)
6. Bloodsuckers keep blood from coagulating by hirudin, an anticoagulant in their saliva.
34.4. Jointed Appendages Evolve (p. 603)
A. Arthropods Have Jointed Appendages
1. There are probably over 1,000,000 species in the phylum Arthropoda.
2. Arthropods are segmented but possess a rigid, jointed exoskeleton.
a. Freely movable jointed appendages make them very successful on land.
b. An exoskeleton is a strong but flexible outer covering composed primarily of chitin.
c. It serves as protection, attachment for muscles, locomotion, and prevention of desiccation.
d. Chitin is a strong, flexible, nitrogenous polysaccharide.
e. Because arthropod exoskeleton is hard and nonexpandable, arthropods must molt (shed) it to grow larger.
1) Before molting, body secretes a larger exoskeleton, which is soft and wrinkled, underneath the old one.
2) After enzymes partially dissolve and weaken old exoskeleton, animal breaks it open and wriggles out.
3) The new exoskeleton then quickly expands and hardens.
3. Some of the segments of arthropods are fused into regions (e.g., a head, a thorax, and an abdomen).
a. In trilobites, which flourished during Cambrian period, there was pair of appendages on each body segment.
b. Modern arthropod appendages are specialized for walking, swimming, reproduction, etc.
(Fig. 34.9a-c, 34.10a)
c. These modifications account for much of the diversity and success of arthropods.
4. Arthropods have a well-developed nervous system.
a. A brain is connected to a ventral solid nerve cord.
b. The head bears various sensory organs.
c. Compound eyes have many complete visual units, each of which collects light independently; lens of each visual unit focuses image on light sensitive membranes of a small number of photoreceptors within unit. (Fig. 34.8a)
d. In simple eyes, a single lens brings image to focus into many receptors, each of which receives only a portion of the image. (Fig. 34.8b)
5. A variety of respiratory organs are used by arthropods:
a. Marine arthropods utilize gills composed of a vascularized, thin-walled tissue specialized for gas exchange.
b. Terrestrial forms have book lungs (e.g., spiders) or tracheae. (e.g., insects).
1) Book lungs are invaginations (lamellae) to serve in gas exchange between air and blood.
(Fig. 34.9d)
2) Tracheae are air tubes that serve as ways to deliver oxygen directly to cells.
6. Metamorphosis is a drastic change in form and physiology that occurs as an immature stage becomes adult.
a. Metamorphosis contributes to the success of arthropods¾ because the larva eats foods and lives in environments different from the adult; competition between immature and adults of a species is reduced.
b. Reduction in competition allows more members of the species to exist at one time.
B. Chelicerates Have Pincerlike Appendages
1. Chelicerates in the subphylum Chelicerata include the spiders, scorpions, ticks, mites, horseshoe crabs, etc.
2. First pair of appendages are chelicerae, second pair are pedipalps, and the next four pairs are walking legs.
a. Chelicerae are appendages that function as feeding organs.
b. Pedipalps are feeding or sensory in function; in scorpions, they are large pincers. (Fig. 34.9a, c)
3. All appendages attach to a cephalothorax, a fusion of the head and thoracic regions. (Fig. 34.9c)
4. The head lacks antennae, mandibles, or maxillae appendages.
5. The horseshoe crab in the genus Limulus is familiar along the east coast of North America.
(Fig. 34.9b)
a. The anterior shield is a horseshoe-shaped carapace with two compound eyes.
b. The long, unsegmented telson projects to the rear.
c. They possess book gills that resemble the pages in a book.
6. Scorpions are arachnids.
a. They are the oldest terrestrial arthropods known from fossils.
b. They are nocturnal and spend most of the day hidden under a log or rock.
c. The pedipalps are large pincerlike appendages; the abdomen ends in a stinger containing venom.
7. Ticks and mites consist of over 25,000 species and outnumber all other arachnids.
a. Ticks are parasites that suck blood and sometimes transmit diseases.
b. Chiggers are larvae of certain mites and feed on the skin of vertebrates.
8. Spiders have a narrow waist separating the cephalothorax from the abdomen.
a. Spiders have numerous simple eyes rather than compound eyes.
b. The chelicera are modified as fangs with ducts from poison glands. (Fig. 34.9c)
c. The abdomen has silk glands and they spin a web to trap prey.
d. Invaginations of the body wall form lamellae (pages) of the book lungs; air flows across the lamellae in the opposite direction from blood flow to exchange gases more efficiently. (Fig. 34.9d)
C. Crustacea Have a Calcified Exoskeleton
1. About 40,000 species of crustacea belong to the subphylum Crustacea.
2. They are mostly marine.
3. The head usually bears compound eyes and five pairs of appendages.
4. The first two are antennae and antennules, lie in front of the mouth and have sensory functions.
5. Three pairs of appendages (mandibles, first and second maxillae) lie behind mouth and are used in feeding.
6. Biramous appendages have two branches; one branch is a gill and the other is the leg branch.
7. Copepods and krill feed on algae; they are numerous and an important link in marine food chains.
8. Barnacles have a thick, heavy shell and are sessile.
a. Stalked barnacles attach by their stalk; stalkless barnacles attach directly to the shell.
b. Barnacles begin life as free-swimming larvae; transform into sessile organisms on wharf pilings, rocks, etc.
c. They extend feathery structures (cirri) to filter feed.
9. Decapods include shrimps, lobsters, crabs, etc. where the thorax bears five pairs of walking legs.
(Fig. 34.10)
a. Gills are above the walking legs.
b. A solid carapace covers the fused head and thorax (cephalothorax).
c. Abdominal segments have a pair of swimmerets, small paddlelike structures.
d. The first pair of swimmerets in a male are stronger to pass sperm to the female.
e. The last tail segments are the uropod and telson, which make a fan-shaped tail.
f. A crayfish awaits prey and uses its claws to carry it to the mouth.
g. The respiratory system consists of gills under the carapace.
h. The stomach has an anterior gastric mill to grind food, and a posterior region where absorption takes place.
i. Green glands in the head region excrete metabolic wastes through a duct at the base of the antennae.
j. The coelom is reduced in arthropods and forms the space about the reproductive system.
k. A heart pumps blood containing hemocyanin into a hemocoel, where the hemolymph flows about organs.
l. A brain is connected to a ventral nerve cord; there are periodic ganglia that give off lateral nerves.
m. Sexes are separate in crayfish.
1) The male has a coiled sperm duct that opens to the outside at the base of the fifth walking leg.
2) The female ovaries open at the base of the third walking legs.
3) A fold between the bases of the fourth and fifth pairs serves as a seminal receptacle.
4) Following fertilization, the eggs are attached to the swimmerets of the female.
D. Uniramia Breathe with Tracheae
1. The subphylum Uniramia includes millipedes, centipedes, and insects.
2. Uniramous appendages attached to the thorax and abdomen have only one branch, the leg branch.
3. Head appendages include only one pair of antennae, one pair of mandibles, and one or two pairs of maxillae.
4. Uniramia live on land and breathe by means of air tubes called tracheae.
E. Centipedes and Millipedes
1. The class Chilopoda includes the centipedes.
a. The body is composed of a head and trunk; it has many segments, and each segment has a pair of legs.
b. They are carnivorous and the head bears antennae and mouthparts with jaws.
2. The class Diplopoda includes the millipedes. (Fig. 34.11)
a. Millipedes have a cylindrical body with many segments fused, with two pair of legs on each segment.
b. They possess more legs than centipedes, although not one thousand, as the name states.
c. Millipedes dwell in soil, feeding on dead organic matter.
F. Most Insects Have Wings
1. Over 900,000 species of insects (superclass Insecta) are known; this is more than all other animal species combined.
2. Most are adapted to life on land but some are secondarily aquatic.
3. The insect body is divided into a head, a thorax, and an abdomen. (Fig. 34.14a) [transp. 185]
a. The head bears sense organs and mouthparts.
b. The thorax bears three pairs of legs and one or two pairs of wings; wings provide many advantages.
c. The abdomen contains most of the internal organs.
4. The exoskeleton of an insect is lighter and contains less chitin than that of many other arthropods.
5. The grasshopper has adaptations as a herbivorous insect.
a. The third pair of legs is suited to jumping.
b. There are two pair of wings; the front wings are protective and leathery, the thin hind pair fold up.
c. Each side of the first abdominal segment has a tympanum for soundwave reception.
d. Two pairs of projections form an ovipositor in females; this is used to dig a hole when laying eggs.
e. The digestive system is complex.
1) The mouth mechanically breaks food, and salivary secretions begin digestion.
2) The crop temporarily stores food.
3) The gizzard finely grinds the food.
4) Digestion is completed in the stomach, and gastric ceca are cavities that assist absorption of nutrients.
f. The excretory system consists of Malpighian tubules.
1) The tubules extend into the hemocoel.
2) Nitrogenous wastes are collected and excreted into the digestive system.
3) Formation of solid nitrogenous wastes (uric acid) conserves water.
g. The respiratory system begins with openings in the exoskeleton called spiracles.
1) Air enters into small tubules called tracheae.
2) Tracheae branch many times until they reach moist areas of gas absorption.
3) Air movement through this tracheal system is assisted by air sacs.
4) Air enters the anterior four spiracles and exits by the posterior six.
5) Tracheae are a factor in limiting the size of insects.
h. The circulatory system contains a slender, tubular heart.
1) The heart lies against the dorsal wall of the abdominal exoskeleton.
2) The heart pumps hemolymph into the hemocoel where it circulates before returning to the heart.
3) Hemolymph is colorless because it lacks respiratory pigment; the tracheal system transports gases.
i. Reproduction is adapted to life on land.
1) Internal fertilization protects gametes from drying out.
2) Female grasshoppers deposit eggs in the ground with an ovipositor.
j. Grasshoppers undergo incomplete metamorphosis, the immature nymph is recognizable as a grasshopper.
k. Other insects undergo complete metamorphosis; wormlike larvae reorganize into adults.
l. Some species (e.g., bees and ants) exhibit colonial social behavior.
m. Entomology, the study of insects, is a major field of biology.