744 ON THE yoUNG STAGES OF ECHINI. 



ing ; the anus is placed near the posterior extremity of the faseiole. The odd 

 ambulacrum carries four or five large tentacles with a lobed disk (PL XJX. 

 f. .'); the pores of the odd ambulacrum arc single, not in pairs; the othei 

 ambulacra are short, straight, well denned, consisting of three and four pairs 

 of pores not \ct conjugated ( /'/. XIX. f. / ). In older specimens the posterior 

 edge of the mouth becomes labiate; the anus approaches the sulianal faseiole. 

 which sends out a rudimentary anal branch (I'l. XJX. /'. ;). eventually 

 uniting with the peiipetalous faseiole, the outline of which becomes more 

 pentagonal, undulating, and elongated with the increasing size of the petaloid 

 ambulacra (PL X/X. /'. ; 9). The posterior vil^r becomes more bevelled 

 with age, the subanal plastron more prominent, tin- lateral pairs of ambulacra 

 gradually 'end to unite 1 I'l. XlX.f. .'<). passing from a strictly Brissopsis out- 

 line to one considered hitherto characteristic of Toxohrissus. The spines in 

 all young Spatangoids are less numeroxis and strikingly larger, in proportion 

 to their size, than in the adult (see I'l. XIV.. AT/., XVII.. MX.). 



In Echinocardium cordatum the changes of the mouth, of the outline of 

 the internal ambulacra! faseiole. and the gradual continence of the Lateral 

 ambulacra, are similar to those of Brissopsis ; the posterior extremity under- 

 goes the greatest change in outline : the sulianal plastron is vwy prominent 

 \ I'l. XIX. J. /;); in fact, the outline of young Echinocardium cordatum re- 

 calls Echinocardium gibbosum. The sulianal faseiole and the anal branch are 

 originally united (PI. XIX. f. /',). but as the specimens Increase in size, the 

 anal branch separates from it (I'l. XIX. f. /;). The odd ambulacra! pores 

 are at first two single rows of pores 1 I'l. XIX. f. /.', I. which by closer crowd- 

 ing eventually alternate (I'l. X/X. f. /<;). but are not arranged in pairs. 



The young Agassizia. a quarter of an inch in length, is a Hat elliptical 

 Spatangoid resembling Gualteria (PI XIV. f. .'/). The peripetalous and 

 lateral fascioles have the same general limits as in the adult ; the arrange- 

 ment of the pores is identical in all the ambulacra ; there is but a single pore 

 for each ambulacral plate (I'l. XIV. f. u) as it exists in the anterior pair 

 and odd ambulacra of the adult ; the ambulacral grooves are not yet formed, 

 the anterior groove alone is slightly indicated; the mouth is not labiate 

 (PL XIV f. to). 



A great number of Spatangoid genera are established upon differences ex- 

 isting in the subanal faseiole, the presence or absence of the anal branch, the 

 depth of the ambulacral grooves, the confluence or distinctness of the lateral 

 ambulacra, — all characters subject to great variation during growth. This 



