ix PHYLUM ECHINODERMATA 421 



there is only a single branched gonad, sometimes imperfectly 

 divided into two, with a duct opening on the dorsal surface not 

 far from the mouth. In the Crinoidea the ovaries and testes 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 rachis running through the arm with a centrally situated 

 genital stolon (axial organ). 



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 larvsD 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. 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 hollow outgrowth, the 

 enteroccele, from which are derived the body-cavity with its enclosing 

 peritoneal membrane, 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 

 coelomic vesicles destined to form the body-cavity. A canal 

 opening on the exterior by a dorsally situated opening, the dorsal 

 pore (sometimes double), is formed by invagination from the surface 

 ectoderm, and comes into relation with a canal arising as an out- 

 growth from the rudimentary ambulacral system to form the 

 foundation of the madreporic canal of the adult. In the Crinoidea 

 five dorsal pores and five canals are developed, but the two sets of 

 structures do not enter into direct communication (see p. 399). 



The part of the enterocoele (hydrocoele} destined to give rise to 

 the ambulacral system, at first rounded, becomes compressed, and 

 subsequently divided round the border into five lobes. Each of 

 these lobes grows outwards to become developed subsequently into 

 one of the five radial ambulacral vessels of the Echinoderm ; 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 surface 

 of the larva uniformly, become restricted to a peri-oral band 

 (Fig. 348, 1, 2 por) surrounding a concave area on which the mouth 

 opens. A smaller adoral band (1, 7, aor) in the interior of the 

 month has the function of attracting nutrient particles. Tlio 

 peri-oral band undergoes characteristic changes in the different 

 classes, ;uil the form of the larva at the same time becomes 



