the Morpliology of the Blastoiclea. 227 



orifice, tlie oral plates being of inconspicuous size and con- 

 fined to the summit, and tlie marked cliange in the disposition 

 of the hydrospires and their method of opening externally. 



In Pentremites it will be remembered the hydrospires are 

 situated internally immediately right and left of each ambu- 

 lacrum, then communicate above with the common hydrospire- 

 canal, which opens externally by means of the pores ranged 

 along each side of the ambulacra, and also through the spi- 

 racles at the apex. But no ])art of the hydrospiral a])]:)aratus 

 is visible externally, the whole of it behig concealed by the 

 lancet-plate and side plates of the wide ambulacra. 



In FhcenoscMsma, on the other hand, the radial sinuses are 

 much Avider and deeper than are those of Pentremites. The 

 lancet-, side, and outer side plates fail to fill them com- 

 pletely, and are confined, generally speaking, to the bottom 

 of the cavity of each sinus. The spaces so left uncovered, 

 consisting of the sides of the sinuses, formed by the inturned 

 edges of the radials and orals, but chiefly, and sometimes 

 wholly, of the former, are occupied by a variable number of 

 subparallel slits, which are in fact the openings of the hydro- 

 spires exposed to view. The slits near the bottom of the 

 cavities are the longest and most completely covered, and 

 those near the top of each sinus the shortest, the outer ones 

 being sometimes visible throughout their entire length. The 

 number of the slits exposed and the amount of their exposure 

 entirely depend on the relative size of the side and outer side 

 plates, and how far they extend in a lateral direction towards 

 the sides of the sinuses. In this way the hydrospiral canal 

 and true spiracles are dispensed with, the hydrospires com- 

 municating directly with the exterior without the aid of any 

 intermediate orifices. 



P/uvnoschi'sma is allied to the genus Codastey, M'Coy, both 

 in the structure of its respiratory organs and also in the 

 absence of ambulacral pores. It differs, Iiowever, from 

 M'Coy's genus in the partial exposure only of the hydrospiral 

 slits, and in their presence in the anal intcrradius, as well as 

 in the four others. PhcvnoscMsma j in consequence, possesses 

 ten groups of hydrospires, whilst Codaster has only eight. 

 Further, the former genus has relatively smaller orals tiian 

 the latter, and it may possess outer side plates to the ambu- 

 lacra. 



Ph(vnoschis)na differs from Oroi^liocrinusm the fact tliat the 

 ambulacra are nowhere in contact Avith the sides of the radial 

 sinuses, as in \o\\ Seebach's genus, and that the oral plates 

 never show in a side view of the calyx. To Pentremilidea 

 Pha'noscliisma stands in the same relation that it does to 



