Phylum Echinodermata and Related Groups 549 



board. No doubt this helped the starfish population considerably. The 

 harm done to the oyster beds was certainly not decreased by this prac- 

 tice. Now more effective control methods are utilized. 



THE CLASS OPHIUROIDEA 



Although the members of this class, the serpent stars or brittle 

 stars (Fig. 183, d), are probably as abundant along the seashore as 

 are the familiar starfish, they are not seen so frequently. This is due 

 to their more secretive habits : they live under rocks, inside sponges, 

 and at times burrowed in the mud. They usually do their feeding at 

 night. 



Unlike the true starfish, the arms are narrowly joined to the 

 central disc and are even more easily broken off and regenerated. All 

 the visceral organs are within the central disc, and the slender snake- 

 like arms have only a small coelom, a nerve cord, a passageway for the 

 water-vascular system, and a small hemal space. Within the arms are 

 articulated ossicles which allow great freedom of movement. The di- 

 gestive system consists only of the mouth and saclike stomach. There 

 is no intestine or anus, and undigestible materials are cast out through 

 the mouth. The madreporite is on the oral surface, and the tube feet 

 are of but limited use in locomotion, rather they are sensory in function. 



The brittle stars are able to swim about by using the arms as 

 propelling organs. More often they move about the bottom by grasp- 

 ing objects and pushing themselves forward or by sort of hopping 

 movements. They eat mainly bottom detritus, and the tube feet do some 

 selection of the food and pass it along to the mouth. 



In nearly all serpent stars, the sexes are separate and fertilization 

 is external. The development of the young roughly parallels that of 

 the starfish; however, the larva is somewhat different, possessing long 

 ciliated arms and is known as a pluteus larva (Fig. 181,5). Some 

 species retain the eggs within a special genital bursa and the young go 

 through their metamorphosis in this structure. In some the upper disc 

 is thrown off to allow the young to escape. 



One peculiar ophiuroidean is the basket star group of the genus 

 Gorgonocephalus (Fig 183,z) in which the arms branch many times 

 and end in small tendril-like tips. 



