227 



whitish. In the Ophiurans the eggs and spermatozoa pass out of 

 the body through little holes between the plates on the under side 

 of the body. In those starfishes in which the alimentary canal 

 is a blind sac, the eggs are emptied into the body cavity ; but 

 how they pass out is unknown. In some starfishes they escape 

 through certain (interradial) plates on the back. In the Eebi- 

 noids they make their exit from between the ambulacra. In the 

 Holothurians, however, there is a duct leading from the generative 

 gland opening out near the mouth, between the tentacles. The 

 eggs are usually round, and minute ; the spermatozoa of the usual 

 tailed form. Fertilization takes place in the water. 



Remembering that there are five well-marked divisions of 

 Echinoderras, i.e., Crinoidea, Ophiuroidea, Asteroidea, J?chinoid< /, 



the mode of development of the respective orders. 



Development of the Crinoids. While we know nothing of the 

 mode of development of the true Pentacrinus and Rhizocrinus, 

 the lineal descendants of the Crinoids of the earlier geological 

 ages, we have quite full information regarding the life-history of 

 the Antedon, which is for a part of its life stalked, and is in fact 



The following account is taken (sometimes word for word) from 

 Professor Wyville Thompson's researches on the Antedon rosaceus 

 of the British coast. The ovaries open externally on the pinnules 

 of the arms, while there is no special opening for the spermatic 

 particles, and Prof. Thompson thinks they are "discharged by the 

 thinning away and dehiscence of the integument." The ripe eggs 

 hang for three or four days from the opening like a bunch of 

 grapes, and it is during this period that they are impregnated. 

 The egg then undergoes total segmentation. Fig. 86, A, represents 

 the egg with four nucleated cells, an early phase of the mulberry 

 or morula stage. After the segmentation of the yolk is finished, I as 

 cells become fused together into a mass of indifferent protoplasm, 

 with no trace of organization, but with a few fat cells in the centre. 

 This protoplasmic layer becomes converted into an oval embryo, 

 whose surface is uniformly ciliated. The mouth is formed, with 

 the large cilia around it, before the embryo leaves the egg. When 

 hatched, the larva is long, oval, and girded with four zones of cilia, 

 with a tuft of cilia at the end, a mouth and anal opening, and is 

 about -8 millimetre in length. The body cavity is formed by an 



