738 ON THE YOUNG STAGES OF ECHINI. 



one in each poriferous zone for each ambulacrum. This extraordinary shape 

 and structure the young do not retain long; they soon become pyriform 

 (PL XII f. 5, o), the blunt extremity being the posterior; the test becomes 

 greatly flattened and the anus approaches the edge. The rosette is now 

 composed of three and two pairs of simple pores in each poriferous zone for 

 each ambulacrum, the anterior ambulacrum having only two pairs in each 

 zone. The tubercles are proportionally smaller, though there are still only 

 two rows in each area, but farther apart. In the next stage ( PL XII. f. ,;, a) 

 we find the rudimentary rosette composed of lour and five pairs of pores 

 close together, and two or three distant pairs of pores, in the following am- 

 bulacral plates, one pair in each plate, which in subsequent stages increase 

 in number and extend almost to the edge of the test. The test has become 

 quite flattened; the lower side is concave, undulating: the ambulacra! zones 

 are now much narrower than the interambulacral ones. Each plate still has 

 only one tuhen-le: the lines of separation between the two zones run 

 straight from the edge of the test to the apex. It is only in somewhat 

 older stages (PL XII. f. w), when the rosette loses its radiating outline, and 

 assumes ;i slightly petaloid shape, that we find the angle formed ill the adult 

 at the hase of the petal in the ambulacra! zone: from this point the ambula- 

 cral plates widen rapidly : each plate now carries from two to six smaller 

 tubercles. The outline is quite pentagonal; the lower surface concave, but 

 little undulating. The anus i- placed neai- the edge, and covered, as in all 

 preceding stages, by a single plate | I'l. XII. f. u); the anal system in older 

 specimens has five plates ( I'l. XII. l'. /.•, /.-). the plate first formed remaining 

 somewhat the largest As tin' young Echinarachnius increases in size its 

 outline becomes more circular, and in specimens measuring one fifth of an 

 inch in diameter has the general appearance of the adult | I'l. XII. f. is). 

 The furrows joining the ambulacra! pores appear scon after the first traces 

 of a true rosette are seen; they become deeper and the pores separate in 

 proportion to the petaloid structure of the abactinal part of the ambulacrum. 

 The tubercles are proportionally much smaller and more numerous, and 

 soon after the ambulacra have a well-developed rosette, and nearly bear to 

 the plates the ratio which they have in the adult. 



Young specimens of Mellita hexapora, measuring ^ of an inch in diameter, 

 are almost circular, with a thickened raised edge, as in Laganuin. and as yet 

 have no lunules ( PL XI. f. l). The rosette is simply a series of radiating 

 pores, three and two in each poriferous zone, for each ambulacrum, extending 



