STARFISH 79 



caeca are two tiny secreting bodies which open into the intestine, 

 so that the nitrogenous wastes leave the body through the anus. 

 It has already been noted that the intestine does not function for 

 the egestion of material from the alimentary canal. It is also com- 

 monly stated that the amoebocytes function as excretory agents in 

 that they ingest various types of solid waste materials. Later 

 these amoebocytes pass through the thin walls of the dermal 

 branchiae and leave the body permanently. 



4. Reproduction 



Unlike the hermaphroditic condition noted in Hydra, the sexes 

 are separate in the Starfish. No external differentiation between 

 the two sexes is to be noted, and, as a matter of fact, there is very 

 little internal structural differentiation except during the breeding 

 season, for the gonads in both sexes are very small during the 

 remainder of the year. In sexually mature animals, in the spring 

 of the year, the gonads of both sexes gradually attain full size and 

 then they fill up a considerable portion of the body cavity in the 

 central disc and extend out into each arm for a considerable 

 distance. The ripe germ cells leave the body through tiny genital 

 pores on the aboral surface of the body — one of which is located 

 near the base of each arm. The fertilization of the Starfish eggs 

 occurs in the surrounding water, and the zygote rapidly develops 

 into a free-swimming, ciliated embryo which is definitely bilateral 

 for a time and then gradually changes into the radially symmetri- 

 cal adult condition. 



5. Nervous System 



Our previous study of Hydra revealed the presence of a nerve 

 net composed of numerous interconnected nerve cells widely 

 distributed throughout the ectoderm layer. In the Starfish there 

 are many such nerve cells with wide distribution, and, in addition, 

 a more highly developed and definite nerve tissue is found in which 

 the constituent elements are grouped to form radiating nerve 

 fibers of various lengths. This central nervous system of the 

 Starfish consists essentially of a nerve ring surrounding the 

 mouth below the ring canal and, in each arm, a fine nerve fiber 

 which, beginning at the nerve ring, runs to the tip of the 

 arm where it ends in a primitive pigmented eye spot. From the 

 nerve ring and the nerve fibers in each arm many small branches 



