ZOOLOGY 



SECT, 



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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 ccelome contains a fluid in which, as in the Starfish, 

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

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

 vibratile 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 ccelomic fluid. 



The part of the ccelome 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 oxygenation of 

 the coelomic fluid en- 

 closed in this com- 

 partment. 



A blood- vascular 

 system is also pre- 

 sent, and has an 

 arrangement corre- 

 sponding to that al- 

 ready described in 



rect 



ati 



FIG. 208. Alimentary canal and other organs of Sea- V 



urchin as seen when the oral half of the corona has n*n,^ it h tile addition 



been removed: />. ,-. vet. aboral ring- vessel of the blood- 1 Q two lll'O'e 



vascular system ; all. alimentary canal ; amp. ampullae; ! . 6 



int. ves. intestinal blood-vessels ; I ant. lantern of Aristotle ; I tmal Vessels (r IP'S. 

 ces. (esophagus; or. r. r. oral ring-vessel of the blood- oci'i i o . no , y 

 vascular system; or. ovary: reet. rectum; .</>/*. siphon. A" aim Zyo, lilt. VCSj. 



(From Leuckart, partly after Cuvier.) The reproductive 



organs consist o 



- five masses of minute rounded follicles (Fig. 298, cv) situated in 

 the anal portion of the shell, and 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 (oyoid gland) are, as alread 

 noticed, aborted in the adult Sea-Urchin. 



The early stages in the development of the Sea-Urchin a 

 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 



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