396 ZOOLOGY SECT. 



occupy a remarkable position, being situated in the dilated bases 

 of the pinnules : but as in the other classes they are connected 

 by means of a genital rt'cJns running through the arm with a 

 centrally situated genital stolon. 



Development and Metamorphosis. A few of the members 

 of each class of Echinoderms are viviparous, in the sense that the 

 development of the young takes place in some sheltering cavity, 

 or brood-pouch, on the surface of the body of the parent. But in 

 most development takes place externally, and the larva? are free- 

 swimming. The ovum in all undergoes regular and nearly equal 

 segmentation, resulting in the formation of a ciliated blastula, 

 which becomes invaginated so as to form a typical gastrula, like 

 that of some Ccelenterata (p. 161). The invaginated cells form the 

 lining membrane (the endoderm layer) of an internal cavity the 

 primitive alimentary cavity or archenteron : the enclosing cells 

 form the ectoderm ; between the endoderm and ectoderm, and 

 derived from the former, appear the cells of the mesoderm or middle 

 layer. From the' archenteron is given off a single or double 

 hollow out-growth, the vaso-pcritoneal vesicle or vesicles, from which 

 are derived the body-cavity with its enclosing peritoneal mem- 

 brane, and the vessels of the ambulacral system with their various 

 appendages. In the Crinoidea the vesicle destined to form the 

 ambulacral system is developed independently of the peritoneal 

 vesicles destined to form the body-cavity. A canal opening on 

 the exterior by a dorsally situated opening, the dorsal pore, (some- 

 times double), is formed 'by invagination from the surface ectoderm, 

 and comes into relation with a canal formed by out-growth from 

 the rudimentary ambulacral system to form the foundation of the 

 madreporic canal of the adult. In the Crinoidea there are formed 

 five dorsal pores and five canals, but the two sets of structures do 

 not enter into direct communication (see p. 375). 



The part of the vaso-peritoneal vesicle (lii/drocode) destined to 

 give rise to the ambulacral system, at first rounded, becomes com- 

 pressed, and subsequently divided round the border into five lobes. 

 Each of these lobes grows outwards to become developed subse- 

 quently into one of the five radial ambulacral vessels of the Echino- 

 derm ; the central part of the hydrocoele gives rise to the ring- 

 vessel surrounding the oesophagus. 



The cilia, which at first (in the gastrula stage) covered the sur- 

 face of the larva uniformly, become restricted to two definite bands, 

 one in front of the mouth or pra?-oral, the other in front of the anus 

 or pra3-anal. These bands undergo characteristic changes in the 

 different classes, and the form of the larva at the same time 

 undergoes modification by the formation, except in the Crinoidea, 

 of variously arranged processes along the course of the ciliated bands. 

 The resulting larva, or Echinopccdium, exhibits always marked 

 bilateral symmetry. 



