392 



ZOOLOGY 



SECT. 



amp 



into it at both ends ; 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 

 amoeboid corpuscles (amcebocytes) with long pseudopodia, and 

 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 coelomic fluid. 



The part of the coelome containing 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 enclosed 

 in this compartment, which is known as the lantern-ccelome. 



The perihsemal and haemal or lacunar systems, as well as 



the axial organ, will 

 be referred to in the 

 account of the general 

 structure of the 

 phylum. 



The reproductive 

 organs consist of five 

 masses of minute 

 rounded follicles (Fig. 

 332, ov) 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 (p. 378) are 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 381. 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 

 calcareous rods. A metamorphosis, in which the bilateral larva 

 becomes converted into the radial adult, takes place as in^ the 

 Starfish. 



FIG. 332. Alimentary canal and other organs of Sea- 

 urchin as seen when the oral half of the corona has 

 been removed, ab. r. ves. aboral ring of the haemal 

 system ; ali. alimentary canal ; amp. ampullae ; int. ves. 

 intestinal blood-vessels ; lant. lantern of Aristotle ; ces. 

 oesophagus ; or. r. ves. oral ring-vessel of the haemal 

 system ; ov. ovary ; reel, rectum ; siph. siphon ; z. 

 teeth. (From Leuckart, partly after Cuvier.) 



