Chapter 33 An Introduction to Invertebrates

Overview: Life Without a Backbone Invertebrates are animals that lack a backbone They account for 95% of known animal species They are morphologically diverse For example, the Christmas tree worm (Spirobranchus giganteus) has tentacles for gas exchange and feeding

Figure 33.1 Figure 33.1 What function do the red whorls of this organism have?

Sponges are suspension feeders, capturing food particles suspended in the water that passes through their body Water is drawn through pores into a cavity called the spongocoel and out through an opening called the osculum Sponges lack true tissues and organs

Choanocyte Osculum Spongocoel Pore Spicules Epidermis Water flow Figure 33.4b Choanocyte Osculum Spongocoel Pore Spicules Epidermis Figure 33.4 Anatomy of a sponge. Water flow Amoebocytes Mesohyl

Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food Sponges consist of a gelatinous noncellular mesohyl layer between two cell layers Amoebocytes are found in the mesohyl and play roles in digestion and structure Most sponges are hermaphrodites: Each individual functions as both male and female

Annelids Annelids have bodies composed of a series of fused rings Annelids are coelomates The phylum Annelida is divided into two groups Polychaeta (polychaetes) Oligochaeta (earthworms and their relatives, and leeches)

Earthworms Earthworms eat through soil, extracting nutrients as the soil moves through the alimentary canal Earthworms are hermaphrodites but cross-fertilize Some reproduce asexually by fragmentation

Leeches Most species of leeches live in fresh water; some are marine or terrestrial Leeches include predators of invertebrates, and parasites that suck blood Leeches secrete a chemical called hirudin to prevent blood from coagulating

Figure 33.25 Figure 33.25 A leech.

Arthropods Two out of every three known species of animals are arthropods Members of the phylum Arthropoda are found in nearly all habitats of the biosphere

Arthropod Origins The arthropod body plan consists of a segmented body, hard exoskeleton, and jointed appendages This body plan dates to the Cambrian explosion (535–525 million years ago) Early arthropods show little variation from segment to segment

Arthropod evolution is characterized by a decrease in the number of segments and an increase in appendage specialization These changes may have been caused by changes in Hox gene sequence or regulation

General Characteristics of Arthropods The appendages of some living arthropods are modified for functions such as walking, feeding, sensory reception, reproduction, and defense

Pincer (defense) Mouthparts (feeding) Figure 33.30 Cephalothorax Abdomen Thorax Antennae (sensory reception) Head Swimming appen- dages (one pair per abdominal segment Figure 33.30 External anatomy of an arthropod. Walking legs Pincer (defense) Mouthparts (feeding)

The body of an arthropod is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin When an arthropod grows, it molts its exoskeleton

Arthropods have eyes, olfactory receptors, and antennae that function in touch and smell Arthropods have an open circulatory system in which hemolymph is circulated into the spaces surrounding the tissues and organs A variety of organs specialized for gas exchange have evolved in arthropods

Molecular evidence suggests that living arthropods consist of four major lineages that diverged early in the phylum’s evolution Chelicerates (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders) Myriapods (centipedes and millipedes) Hexapods (insects and relatives) Crustaceans (crabs, lobsters, shrimps, barnacles, and many others)

Most modern cheliceriforms are arachnids, which include spiders, scorpions, ticks, and mites

Scorpion Dust mite Web-building spider 50 m Figure 33.32 Figure 33.32 Arachnids. Web-building spider

Arachnids have an abdomen and a cephalothorax, which has six pairs of appendages: the chelicerae, the pedipalps, and four pairs of walking legs Gas exchange in spiders occurs in respiratory organs called book lungs Many spiders produce silk, a liquid protein, from specialized abdominal glands

Myriapods Myriapods are terrestrial, and have jaw-like mandibles Subphylum Myriapoda includes millipedes and centipedes Myriapods are terrestrial, and have jaw-like mandibles Millipedes eat decaying leaves and plant matter Millipedes have many legs, with two pairs per trunk segment

Figure 33.34 (a) Millipede Figure 33.34 Myriapods. (b) Centipede

Centipedes are carnivores Centipedes have one pair of legs per trunk segment

Insects Subphylum Hexapoda, insects and relatives, has more species than all other forms of life combined They live in almost every terrestrial habitat and in fresh water The internal anatomy of an insect includes several complex organ systems

Abdomen Thorax Head Compound eye Antennae Heart Dorsal artery Crop Figure 33.35 Abdomen Thorax Head Compound eye Antennae Heart Dorsal artery Crop Cerebral ganglion Anus Vagina Figure 33.35 Anatomy of a grasshopper, an insect. Malpighian tubules Ovary Tracheal tubes Nerve cords Mouthparts

Insects diversified several times following the evolution of flight, adaptation to feeding on gymnosperms, and the expansion of angiosperms Insect and plant diversity declined during the Cretaceous extinction, but has been increasing in the 65 million years since

Flight is one key to the great success of insects An animal that can fly can escape predators, find food, and disperse to new habitats much faster than organisms that can only crawl

Many insects undergo metamorphosis during their development In incomplete metamorphosis, the young, called nymphs, resemble adults but are smaller and go through a series of molts until they reach full size

Insects with complete metamorphosis have larval stages known by such names as maggot, grub, or caterpillar The larval stage looks entirely different from the adult stage

(a) Larva (caterpillar) (b) Pupa (c) Later-stage pupa (d) Emerging Figure 33.37 (a) Larva (caterpillar) (b) Pupa (c) Later-stage pupa Figure 33.37 Complete metamorphosis of a butterfly. (d) Emerging adult (e) Adult

Most insects have separate males and females and reproduce sexually Individuals find and recognize members of their own species by bright colors, sound, or odors Some insects are beneficial as pollinators, while others are harmful as carriers of diseases, or pests of crops Insects are classified into more than 30 orders

Crustaceans While arachnids and insects thrive on land, crustaceans, for the most part, have remained in marine and freshwater environments Crustaceans, subphylum Crustacea, typically have branched appendages that are extensively specialized for feeding and locomotion Small crustaceans exchange gases through the cuticle; larger crustaceans have gills

Most crustaceans have separate males and females Isopods include terrestrial, freshwater, and marine species Pill bugs are a well known group of terrestrial isopods Decapods are all relatively large crustaceans and include lobsters, crabs, crayfish, and shrimp

Figure 33.39a Figure 33.39 Crustaceans. (a) Ghost crab