Invertebrates with and Without Brains Sponges (phylum Porifera) are invertebrates with no brain or nervous system of any sort, in either the sedentary adults or the free-swimming larvae. Stimuli received at the body surface produce responses (movements) directly, over the entire body, in these and related lower metazoan animals. Other primitive invertebrates such as hydra, jellyfish, corals, and sea anemones (phylum Cnidaria) have one or more nerve nets. For these radially symmetrical animals, food or danger can come from any direction in the water, and the meshlike nervous system can respond directly without a central control region. Some jellyfish also have a nerve ring that helps coordinate their movements, but no brain. Bilaterally symmetrical invertebrates include the flatworms (phylum Platyhelminthes), roundworms (Nematodes), mollusks (Mollusca), segmented worms (Annelida), and insects and their relatives (Arthropoda). Most of these show cephalization, the presence of an anterior head containing the main processing center of the nervous system, specialized sensory receptors, and the mouth. Echinoderms (Echinodermata) such as starfish are bilaterally symmetrical as larvae, but develop radial symmetry as adults, when they lack a head. Some mollusks are not symmetrical as adults, despite the bilateral symmetry of the larvae. Among the flatworms, some have only nerve nets like those of cnidarians, while more complex planarians, tapeworms, and flukes generally have one or more pairs of ladderlike longitudinal nerve cords with ganglia at the head. These ganglia are clusters of cell bodies of neurons, the most primitive form of a brainlike structure. Nematodes or roundworms have a nerve ring and anterior ganglia organized around the anterior digestive tract, with nerve cords extending toward the head and tail from this center. Mollusks include clams and oysters (class Bivalva), snails and slugs (Gastropoda), and octopuses and squid (Cephalopoda). These animals have nervous systems that vary from simple and relatively uncephalized nerve rings and nerve cords, to a more centralized system with at least four pairs of ganglia. Octopuses and squid have the most complex nervous systems of the mollusks and are the most intelligent invertebrates. The relatively large cephalopod brain contains many clustered or fused ganglia that manage sensory information from complex eyes and produce motor instructions for extremely rapid muscular responses. Giant nerve fibers in squid are the largest neurons known in any animal, up to one millimeter in diameter in a single cell, and are able to conduct rapid impulses that allow lightning-fast movements. Extensive studies of these neurons' structure and function have provided scientific insights that are also applicable to human neurons. Gastropods and cephalopods may show extremely complex behaviors, such as homing, territoriality, and learning. An octopus can have as many as thirty functional brain centers, some of which are memory banks used for experiential learning. Annelids such as earthworms and leeches have paired cerebral ganglia near the mouth, connected by a solid ventral nerve cord to smaller paired ganglia in each body segment. Giant nerve fibers in the nerve cord allow rapid responses to escape from threats using reflex actions and patterned behavior. Earthworms can be taught to travel a maze by simple associative learning, in which repeated stimuli become linked to a specific behavior pattern, but this learning requires many repetitions and disappears within a few days if not reinforced. Arthropods include spiders, scorpions, ticks, and mites (class Arachnida), lobsters, crabs, and shrimp (Crustacea), and insects (Insecta). The nervous system in arthropods is similar to that of annelids in its segmentation, but it is much more complex, and the anterior ganglia tend to be fused into a true brain. Many arthropods have giant neurons like those of some mollusks and annelids, capable of rapid nerve impulse transmission for efficient muscle control. Insects in particular, especially ants and bees, are capable of complex learning and very intricate social behavior. Habituation allows individuals to learn to ignore repeated stimuli that do not produce harmful effects, and cockroaches and ants can learn to run mazes. Sea urchins, sand dollars, sea stars (starfish), and sea cucumbers are echinoderms, in which the bilaterally symmetrical larvae develop a secondary radial or biradial symmetry as they mature. The resulting radial nervous system is not greatly centralized, as there is a mouth but essentially no head. The nervous system consists of a nerve ring around the mouth that is connected to radial nerves and a nerve net. Thus, behavior generally involves only localized responses to stimuli, as along one arm of a starfish.
Scales Scales have evolved in most vertebrate groups to provide a layer of protection for the integument, the outer layers of tissue that protect the internal environment of the animal from the external world. Scales are hardened plates that are made either of bonelike substance or the protein collagen, both of which are formed in the dermis (the dense, connective tissue layer of the skin) or from other rigid proteins such as keratin, which are secreted by epithelial cells in the outer layer of the skin. The protection afforded by scales does not come at the expense of flexibility, however, for most scales are attached only at one edge to folds in the skin and thus form overlapping plates which can slide over each other as the animal moves. As such, scales provide a protective armor that is lighter and more flexible than the armor formed by large plates of dermal bone such as those seen in extinct fishes such as the Placodermi.
