BIOLOGY OF THE SEA-HOW THEY LIVE 47 



system) that a given species has and upon the environment in which it is able 

 to live. It is able to live in a particular environment only if the environment 

 continues to provide the stimulus (light, food, temperature, etc.) to which its 

 receptors are capable of responding. The higher vertebrate animals (mammals) 

 have advanced structural equipment (well-developed nervous system, sensory 

 receptors, and action equipment) which, in contrast to invertebrates, is well 

 integrated, making possible complex behavior patterns and modifications of 

 behavior by learning from previous experience. 



Behavior of invertebrates varies, depending upon the efficiency of their sense 

 organs, the type of nervous system, the kind of action equipment possessed, 

 and it is also dependent upon the animal's past experience and inherited instincts. 

 The intelligence of a species is determined by the degree to which sensory 

 receptors cooperate with one another in governing the nature of behavior. 



The marine sponge, Sycon (phylum Porifera), is one of the lowest of 

 invertebrates. After its free-swimming larva stage, it attaches itself to the bottom 

 and becomes immobile, filtering its food from the surrounding water. There is 

 no nervous system. There is only protoplasmic transmission of impulses from 

 one cell to a neighboring cell, each cell serving the combined functions of action 

 equipment and receptor. Pore cells are stimulated by water currents and 

 chemicals, which start the beating of flagellated cells found lining the body 

 walls. This animal has no other sensory receptor and its stationary existence 

 makes it completely dependent on its environment. 



The sea anemone (phylum Coelenterata), more highly organized, shows an 

 advance in sensitivity to its environment far greater than that of the sponge. 

 It is able to move if the food supply becomes scarce, and it grasps its food by 

 its tentacles and passes it to the mouth. These tentacles have contact sense and 

 chemical sensitivity. The anemone also shows a reaction to light. The actions 

 of coelenterates are integrated by the most primitive tvpe of nervous system 

 known, the nerve net (/rg. 13^. 



The brittle starfish is part of a group called "echinoderms," which include the 

 starfishes and sea urchins (phvlum Echinodermata). They are slightly higher 

 in psychological position than the sea anemone because of their more efficient 

 type of nervous system involving a nerve ring Qfig. 13^. The nerve ring is not 

 a controlling center like a true brain but is a more efficient transmitter of 

 impulses than the nerve net. This nerve ring is used mostly for controlling 

 locomotion, and cooperation between parts is slightly better than in the coelen- 

 terates. These advances, although the echinoderms have radial symmetry, 

 usually allow whatever area is more strongly stimulated to take the lead in 

 locomotion. Brittle stars are sensitive to light, moving about at night capturing 

 their food. The light-sensitive cells are found at the ends of the arms. The sea 

 urchin has light-sensitive pigmented spots on the body. Chemical-sensitive buds 

 are found on the tentacles, tube feet, and the mouth area of brittle stars. The 

 whole body is somewhat chemically sensitive with the ventral side slightly 

 more so. Contact sensitivity is well developed and is important for locomotion. 

 Contact receptor cells have hairlike projections found on the tube feet and 

 tentacles. The sea urchin relies on the spines for its contact responses. Most 

 starfish move about with the action of the tube feet. The britde star moves 

 by the action of its arms, which are powered by large muscles. The sea urchin, 

 when excited, moves by the spines. Otherwise it uses its tube feet. 



