166 INVERTEBRATE ZOOLOGY 



transformation from the one type of symmetry to the other will 

 be discussed in detail. 



Development of the Water -vascular System. — During early 

 larval development, an echinoderm is distinctly bilateral in 

 form (Fig. 84). Radial arrangement of the parts characteristic 

 of the adult makes its appearance only following the formation 

 of the mesoderm. Mesothelial sacs (Fig. 82) are formed as lateral 

 outpocketings of the entoderm just as in embryos of many other 

 groups. Both the right and the left mesoderm pouches undergo 

 a constriction (A) which separates each into an anterior and a 

 posterior sac. The posterior sacs continue to increase in size 

 and ultimately form (C) the right and left coelomic cavities. 

 The anterior sacs are the rudiments of the water-vascular system 

 and are termed the hydrocoel sacs. The left hydrocoel acquires 

 communication with the body wall through a tubular outgrowth 

 (B), which becomes the stone canal of the water-vascular system. 

 The right hydrocoel fails to develop but remains vestigial and 

 finally disappears. The entire water- vascular system is thus 

 formed from the left hydrocoel. As it increases in size, it encircles 

 the esophagus of the larva and becomes the ring canal of the 

 vascular system. Five radial pouches extend outward from this 

 ring canal and as they increase in length they become recogniz- 

 able as the radial vessels one of which passes along each arm of 

 the adult. The water-vascular system thus has its origin from 

 one of the mesodermal pouches of a bilaterally symmetrical larva 

 which in later development assumes a radial arrangement of its 

 parts. 



The skeletal system is one of the most characteristic features 

 of the echinoderms. Though details of arrangement and extent 

 of development are highly variable, certain of the plates are 

 fairly constant in their fundamental relations throughout a num- 

 ber of classes. The skeletal plates have their origin in the 

 mesoderm and lie near the surface of the body directly beneath 

 the outer body covering. Spines, frequently associated with 

 these plates, suggest the meaning of the name Echinoderma 

 which is Latin for spiny skin. In some instances (Echinoidea), 

 the plates are rigidly articulated to form a continuous shell or 

 test, unchangeable in form, within which most of the organs lie. 

 In other classes (Asteroidea, Ophiuroidea, and Crinoidea), at least 

 some of the plates are movable and permit of some flexibility in 

 the parts which they cover. Among the Holothuroidea, the 



