PHYLUM ECHINODERMATA 



311 



long looped intestine, the posterior end of 

 which is a muscular enlargement called the 

 cloaca ending at the posterior anus. The 

 food of most sea cucumbers consists of 

 organic particles extracted from the sand or 

 mud which is taken into the digestive tract. 

 From 10 to 30 of the tube feet surrounding 

 the mouth are modified as tentacles for 

 procuring food. Respiration is carried on 

 through the cloaca. Connected to the cloaca 

 are two long-branched tubes, respiratory 

 trees. The muscular cloaca pumps water in 

 and out of the tree, which serves both as a 

 respiratory and excretory organ. The cloaca 

 and respiratory trees also function as ex- 

 cretory organs. 



Some sea cucumbers are long, slender, and 

 wormlike. Their colors are varied— brown, 

 yellowish, reddish, whitish, black, pink, 

 purplish, etc. One Puget Sound species, 

 Psolus chitinoides, has an orange-colored 

 body and crimson-colored neck and tenta- 

 cles. 



Sea lilies or feather stars 



These (Fig. 190) are called crinoids. In 

 some species they are attached to the sea 

 bottom by a long jointed stalk. Their rays, 

 5 or 10 in number, are often branched near 

 the base and bear smaller branches called 

 pinnules along their sides, giving them a 

 feathery appearance. About 630 living species 

 of crinoids are known. They live in both 

 shallow and deep water. Fossil remains of 

 crinoids are very abundant in some lime- 

 stone formations. 



Autotomy 



Both starfishes and sea cucumbers prac- 

 tice autotomy. Starfishes break off injured 

 arms at a particular point; and the sea 

 cucumbers, Thyone, when irritated, may, by 

 violent contractions, cast out through the 

 cloacal opening most of the viscera; only the 

 ends are left inside to regenerate. In both 



cases, the lost parts are soon regenerated. 

 One investigator found that of 150 evis- 

 cerated sea cucumbers {Thyone), all but 5 

 lived and replaced the parts successfuhy. 

 Starfishes with regenerating arms as shown 

 in Fig. 197 are often encountered in nature. 



Artificial parthenogenesis 



The eggs of echinoderms have been used 

 extensively for the study of artificial par- 

 thenogenesis, that is, the activation of de- 

 velopment in eggs by artificial means instead 

 of by the penetration of spermatozoa. Loeb 

 succeeded in obtaining actively swimming 

 embryos from sea-urchin eggs by changing 

 the chemical constitution of the sea water. 

 Later it was found that the eggs of various 

 species of animals, particularly those of 

 starfishes, sea urchins, and frogs, could be 

 induced to develop when subjected to a 

 number of agents including heat, acids, and 

 potassium and sodium chloride. Electrify- 

 ing, shaking, and pricking some eggs with 

 a needle also stimulate development. The 

 efficacy of the agent differs for different 

 types of eggs, each type responding to one 

 agent more readily than to others. Artificially 

 stimulated eggs may give rise to embryos 

 and larvae; and in some cases, such as the 

 starfishes, sea urchins, and frogs, they may 

 produce adult animals. Even fatherless rab- 

 bits have been obtained by stimulating un- 

 fertilized rabbits' eggs to develop, and then 

 replacing them in female rabbits. Thus eggs 

 that normally require union with spermato- 

 zoa to initiate development may give rise to 

 mature animals just as do parthcnogenetic 

 eggs. 



ORIGIN AND RELATIONS 

 OF THE ECHINODERMATA 



Echinoderms and coclenterates, because 

 of their radial symmetn,', were at one time 

 placed together in a group called Radiata. 



