430 ZOOLOGY SECT. 



Ophiuroidea there are five pairs of gonads, a pair in the walls 

 of each of five genital Imrsw, which open on the exterior by slits 

 on the oral surface close to the mouth. In the Holothuroidea 

 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 Oinoidea 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. 



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

 most, development takes place externally, and the larvae 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 Coelenterata (p. 173). 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 

 enteroccde, 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 outgrowth from the rudimentary am- 

 bulacral 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. 408). 



The part of the enterocoele (hydroccde) 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 hydrocrele gives rise to the ring-vessel sur- 

 rounding the oesophagus. 



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

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

 (Fig. 343, ^>or) surrounding a concave area on which the mouth 



