ECHINODERMATA. 



FIG. 10. OVUM OF Toxo- 



T 



JECTIONS OF THE PROTOPLASM 

 PENETRATING THE ZONA RADI- 

 ATA ^ r ^ (After Selenka.) 



developed, and therefore remains in- 

 complete. The perforation so formed, 

 becomes the micropyle of the Holo- 

 thurian egg, which was first discover- 

 ed by Joh. Miiller. The albuminous 

 membrane just described for Holo- 

 thurians is also found in Asteroids 

 (fig. 5) and Echinoids. In these 

 groups there is no proper micropyle, 

 though in Ophiothrix a nutritive pas- 

 sage perforates the membrane at the 

 attachment of the ovum before the 

 period when the ovum becomes free 

 (Ludwig). The formation of the zona 



radiata has been studied by Selenka. It is secreted by the 

 protoplasm of the ovum, and has a gelatinous consistency, and 

 after it is formed the peripheral layer of the protoplasm of 

 the ovum sends out through it pseudopodia-like processes to 

 absorb nutriment from without. These 

 processes are at first large and irregular, 

 but soon become finer and finer (fig. 10), 

 and acquire a regular radiating arrange- 

 ment. They are withdrawn when the ovum 

 is ripe, but they nevertheless give rise to 

 the finely radiated appearance of the mem- 

 brane, the radii being in reality delicate 

 pores. 



In the Crinoids the generative rachis 

 consists of a tube, the epithelium of which 

 is formed of the primary germinal cells. 

 (Fig. 11.) While some of these cells en- 

 large and become ova, the remainder supply FIG. n. TRANSVERSE 



, r - ... , -,11- SECTION THROUGH THE 



the elements for a follicular epithelium, PINNA O p A SEXUALLY 



which is established round the ova, exactly MATURE COMATULA. 



(From Gegenbaur, after 

 as in Holothunans. Ludwig.) 



/. Tentacle, g. Lumen of genital rachis. w. Water-vascular vessel. . Nerve 

 cord. b. Blood-vessel on nerve cord and round genital rachis. eg. Genital canal. 

 td. Dorsal section of the body cavity, rv. Ventral section of body cavity. 



