368 ESSENTIALS OF ZOOLOGY 



extent also. There is a certain amount of diffusion of dissolved 

 wastes through the dermal branchiae and the walls of the tube feet. 

 The nervous system is radially arranged about the oral ring which 

 encircles the mouth just orally to the ring canal. From the oral 

 ring, a radial nerve extends the length of each arm and ends in the 

 pigmented eyespot. These nerves lie in the roof of the ambulacral 

 grooves. The aboral surface is supplied by a less conspicuous aboral 

 nerve which extends from an anal nerve ring. Branches of these 

 nerves extend to the numerous nerve cells distributed in the epi- 

 dermis above the nerve cords. The pigmented eyespots at the tips 

 of the arms are photosensitive and sensitive to touch. The pedi- 

 cellariae and tube feet are also sensitive to touch. There is little 

 centralization except in the oral ring and radial cords, still there 

 is sufficient centralization for the necessary coordination exercised 

 by the animal. 



Reproduction and Life Cycle 



The starfish is dioecious; i.e., the sexes are separate. The repro- 

 ductive systems of the two are similar and each consists of five 

 paired gonads lying in the cavity of the rays beside the pyloric 

 caeca. They open to the exterior by pores in the angles between 

 arms. Mature eggs produced in ovaries of females and mature 

 spermatozoa discharged from testes of males are freed in the ocean 

 water where they unite in fertilization. Total, equal cleavage is the 

 type of division which follows fertilization, and this finally gives rise 

 to the many-celled, free-swimming, ciliated hlastula. The wall of this 

 infolds to form a gastrula. Following this the rounded body becomes 

 somewhat elongated and lobed. Ciliated bands develop over its sur- 

 face and it is known as hipinnaria. This larval stage has bilateral 

 symmetry, and the larva swims about near the surface for weeks 

 by the aid of its ciliated bands. A later modification of the bipin- 

 naria in which there are several extended symmetrical processes, 

 is known as the hrachiolarian stage. Following this condition is a 

 metamorphosis during which many processes are formed, and the 

 radial symmetry superimposes the bilateral. The presence of the 

 bilateral symmetry in these larval stages seems to indicate that the 

 ancestors of echinoderms were likely animals with this type of 

 symmetry. 



