48 



UNDERWATER GUIDE TO MARINE LIFE 



The worms (phylum AnneHda) have a definite advantage over the starfishes 

 and sea urchins in that the head end of the body is well supplied with sense 

 organs, there being progressively less sensitivity toward the posterior end. This 

 permits the head to take the lead in behavior and makes possible a more complex 



SAN OUORM, Nereis, 

 NCRV0U5 SXSTEM 



CRUSTACEAN 

 NERVOUS SYSTEMS 



STARFISH 

 MCRVC RING 



pr.» 



SQUID 

 NERVOUS S'ySTEM 



FISH 

 HtRVOOS 5YSTCM 



OUFAtTORY 

 3ENSE 



Fig. 1 3 . Nervous systems and sense organs. 



adjustment to environment. The sandworm, Nereis, is an example of this group. 

 It burrows into sand and has its chemoreceptors (smell, taste) mainly on the 

 head end of the body, allowing a more directed food-locating behavior than is 

 present in echinoderms. One other factor important in this group for adaptive 

 orientation is the development of a centralized nervous system involving a brain 

 Cfig- 13). These improvements of nervous structure with concentration on the 

 head end integrate the body processes in a better way than the lower animals 

 previously discussed. The marine worms have paired ocelli (simple eyes) which 

 are sensitive to light, and the worms react by hiding by day. Chemical sensitivity 

 is mainly located on the tentacles on the head end although there is some 

 sensitivity on the tail and body appendages. There is strong sensitivity to 

 vibration from the substrate. 



Some members of the phylum Mollusca (snails, octopuses, and squids, but 

 not clams and oysters) have a much better-developed head end, and this anterior 

 part of the body exercises greater control over other parts of the body than that 

 found in the worms. This superiority is also evident in improved sense receptors 

 and nerve conduction. The groups of this phylum have a great variety of 

 behavior, which ranges from the permanently attached oyster and the slow- 

 moving clam to the fast-moving octopus and the squid. Chemical sensitivity is 

 present in the tentacles of the snail and scallop and in the sucking discs of the 



