400 ZOOLOGY SECT. 



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 branchia3 on the 

 peristome is evidently the oxygenation of the coelomic fluid 

 enclosed in this compartment, which is known as the lantern- 



The perihaemal 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. 327, ov) situated in the anal portion of the 

 shell, and each communicating with the exterior by its duct, 



ves 



reef 



.orrves 

 la.nl 



aU 



FIG. 327. Alimentary canal and other organs of Sea-urchin as seen when the ora half 

 of the corona has been removed, ab. r. ves. aboral ring of the ha?mal system ; all. ali- 

 mentary canal ; amp. ampullae ; int. ves. intestinal blood-vessels ; lant. lantern of Aristotle ; 

 oes, oesophagus ; or. r. v. oral ring-vessel of the haemal system ; ov. ovary ; rect. rectum ; sip/i. 

 siphon ; z. teeth. (From Leuckart, partly after Cuvier.) 



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. 385) 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 388. The bilateral larva 

 of the Sea-urchin, which is termed a plutevs, 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. 



