CHAPTER 21 



OJI 



Phylum 



Echinodermata. 



Starfishes, 



Sea Urchins, 



Sea Cucumbers, 



Sea Lihes, and Others 



HE echinoderms, which are spiny-skinned, 

 consist of the starfishes, brittle stars, sea 

 urchins, sea cucumbers, and sea lihes (Fig. 

 190). They are all marine animals and con- 

 stitute a considerable proportion of the ani- 

 mal life of the seashore. The starfish is the 

 best-known type, but the sea urchin and sea 

 cucumber are also quite well known to the 

 seashore visitor. 



The echinoderms are of particular inter- 

 est because: (1) they change from a bilater- 

 ally symmetrical larva to a radially sym- 

 metrical adult; (2) they have remarkable 

 powers of autotomy (self-mutilation) and 

 regeneration of lost parts; (3) the eggs are 

 especially suitable for extensive experimenta- 

 tion on artificial parthenogenesis; (4) they 

 have a water-vascular system which includes 

 organs known as tube feet, and (5) they 

 have internal skeletons of calcareous plates. 

 There are 5 classes of living echinoderms. 



ASTERIAS-A STARFISH 



Structure 



The starfish does not resemble a fish, and 

 a more appropriate name would be "sea- 

 star" because of its habitat and shape; but 

 the name "starfish" is the one by which this 

 animal is best known. On the upper or 

 aboral surface are many spines of various 

 sizes, pedicellariae at the base of the spines, 

 a madreporite which is the entrance to the 

 water-vascular system, and the anal open- 

 ing. On the oral surface are a mouth cen- 

 trally situated in the membranous peristome 

 and 5 grooves (ambulacral), 1 in each ray, 

 from which 2 or 4 rows of tube feet extend. 



The skeleton is made up of calcareous 

 plates or ossicles (Fig. 193), bound together 

 by muscle and connective tissue fibers. The 

 spines are short and blunt and covered by 

 epidermis. Around their bases are many 

 whitish modified spines called pedicellariae. 

 These are little jaws; they look like tiny 

 scissor-blades mounted on a stalk, which, 



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