BIOLOGY OF THE SEA-HOW THEY LIVE 49 



octopus and the squid. The snail has hght-sensitive receptors at the end of 

 the tentacles. The octopus and the squid have eyes that are almost as highly 

 developed as those of the mammals. This has been one of the chief reasons for 

 attributing a high intelligence to these animals. These highly developed receptors 

 and a well-developed nervous system (fig. 13^ place the octopus and the squid 

 among the highest in intelligence among invertebrates, but their apparent lack 

 of sensory and motor integration does not warrant placing them higher than 

 fishes in psychological standing. The scallops have a row of light-sensitive 

 receptors on the mantle edge near the base of the tentacles. Clams have light- 

 sensitive cells on the inner surface of the siphon. An elongated muscular foot 

 in clams provides locomotion. Sea slugs and snails have a gliding movement 

 caused by waves of muscular contractions on the foot. The octopuses move on 

 the ground by using their tentacles and in open water by jet action of expelled 

 water. 



The phylum Arthropoda is a very diverse group composed of almost a 

 half-million species, of which crabs, shrimps, and lobsters are part of a group 

 called "crustaceans." This group lies above the Mollusca in psychological 

 standing because of the existence of a brain and the ladder-type nervous system, 

 which has a pair of ganglia in each body segment. This results in better integra- 

 tion of the parts of the body than is found in molluscs. The head with the 

 specialized sense receptors plays an important part in the animal's orientation. 

 The crustaceans have chemoreceptors in the antennules, or small antennae. 

 These receptors are used to locate food by picking up juices diffusing in water 

 from some distance away. They are also used to detect food by contact. Substrate 

 vibrations are picked up with legs and body. Most of the crustaceans have 

 compound eyes, which are especially well adapted for picking out moving 

 objects. 



Fishes 



Much of the behavior of the invertebrates is closely related to the extent 

 of the development of their nervous system. In the vertebrate fishes, as well as 

 in the invertebrates, the nervous system is principally a bridge between the 

 sense organs and action equipment, but it is distinguished from the invertebrate 

 system by having a spinal cord, which runs along the back, associated with a 

 much better brain (/ig. i3). This tubular conductor permits a unification of 

 the action equipment not possible in the invertebrate ladder-type system. 

 Although fishes have this advanced tubular system, they still retain a form 

 of segmental system, which acts like a ladder-type system (not, however, 

 evolutionarily related to the ladder system) and which accounts for some of 

 the stereotyped behavior found in fishes. This segmental system is retained in 

 part by all higher vertebrate animals. 



SENSE RECEPTORS OF FISHES IN GENERAL 



VISUAL 



The eyes have a flattened cornea with a fixed-focus lens. Vision is usually 

 20-30 per cent binocular because of the position of the eyes on the sides of the 



