PHYSIOLOGY OF THE INVERTEBRATA. i,\r 



destined to furnish the vitellus and spermatozoa with phos- 

 phorus — an element which is present in large quantities in 

 the genital products.* 



In the Grinoidea, the sexes are distinct ; and the tubular 

 genital organs are situated under the perisoma of the 

 pinnulfe. In Antedon the ovaries open externally on the 

 pinnulse, and the ova are discharged by the dehiscence of 

 the integument of the pinnula ; but before leaving the 

 female Antedon the ova remain attached to the opening of 

 the integument for a few days ; during that time they are 

 fecundated. " The testis develops no special aperture, but 

 the spermatozoa appear to be discharged by the dehiscence of 

 the integument." The development of the embryo Antedon 

 is as follows : After the ovum is impregnated it undergoes 

 total segmentation — i.e., a morula is formed. It then passes 

 into a gastrula stage, hatching as a barrel-shaped larva with 

 four bands of cilia. This larva passes through a metamor- 

 phosis, and ultimately becomes a fixed pentacrinus-like form 

 — i.e., Antedon is stalked when young. After some time, 

 however, it separates from its stalk and moves about inde- 

 pendently. 



Professor Huxley says that " on comparing the facts of 

 structure and development, which have now been ascertained 

 in the five existing groups of the Hchinodermata, it is obvious 

 that they are modifications of one fundamental plan. The 

 segmented vitellus gives rise to a ciliated morula, and this, 

 by a process of invagination, is converted into a gastrula, 

 the blastopore of which usually becomes the anus. A mouth 

 and gullet are added, as new formations, by invagination of 

 the epiblast. The embryo normally becomes a free Mhino- 

 pcedium, which has a complete alimentary canal, and is 

 bilaterally symmetrical. The cilia of its ectoderm dispose 

 themselves in one or more bands, which surround the body ; 

 and, while retaining a bilateral symmetry, become variously 

 modified. In the Holothuridea, Asteridea, and Grinoidea, 



* Similar concretions are found in the renal organs of many Invertebrates. 



2 D 



