368 



ZOOLOGY 



SECT.. 



mesentery, passing out from it to the inner surface of the shelL 

 It gives off a short blind diverticulum, the siphon (siph) ; this, 

 together with the intestine itself, probably acts as an organ for the 

 respiration of the coelomic fluid. 



The coelome contains a fluid in which, as in the Starfish, 

 there are numerous corpuscles. Of these there are two kinds, 

 (1) amoeboid corpuscles (amcebocytcs) with long pseudopodia, (2) the 

 mbratile corpuscles, which closely resemble sperms, having a rounded 

 head and a slender vibratile tail : the latter aid in bringing about 

 a constant circulation of the coelomic fluid. 



The part of the coelome in front of Aristotle's lantern is com- 

 pletely cut off from the rest by the arrangement of the membrane 



enclosing the lantern, 

 and the function of 

 the branchiae on the 

 peristome is evidently 

 the pxfge nation of 

 the c^l'omic fluid en- 

 closed in this com- 

 partment. 



^blood- vascular 

 system is also pre- 

 sent, and has an 

 arrangement corre- 

 sponding to that al- 



ves 



rcct 



all 





Siptl, 



alt, 



FIG. 298. Alimentary canal and other organs of Sea-. ' 

 urchin as seen when the oral half of the corona has 

 been removed : al>. ,: i-m. aboral ring-vessel of the blood- 

 vascular system ; nil. alimentary canal ; am/i. ampulln 1 ; 

 int. ves. intestinal blood-vessels ; Innt. lantern of Aristotle; 

 oes. (esophagus; o,: r. r. oral ring-vessel of the. blood- 

 vascular system; o<: ovary; ,--i-t. rectum; xiph. siphon. 

 (From Leuckart, partly after Cuvier.) 



ready described in 

 the case of the Star- 

 fish, with the addition, 

 of two large intes- 

 tinal vessels (Figs. 

 297 and 298, int. res'). 

 The reproductive 

 organs consist of 



five masses of minute rounded, 'follicles (Fig. 298, <,r) situated in 

 the anal portion of the shell,-*ind each communicating with the 

 exterior by its duct, which perforates the corresponding genital 

 plate. The sexes are distinct ; as in the Starfish, there is little- 

 difference to be observed between the ovaries of the female and 

 the testes of the male until we come to examine their microscopic 

 structure. The genital rachides which in the Starfish connect 

 the gonads with the: genital stolon (ovoid gland) are, as already 

 noticed, aborted in the adult Sea-Urchin. 



The early stages in the development of the Sea-Urchin are' 

 very similar to the corresponding stages in the development 

 of the Starfish described on page 359. The bilateral larva 

 of the Sea-Urchin, which is termed a Pluteus, is provided with 

 a number of elongated arms or processes supported by delicate 



