ECHINODERMATA—PELMATOZOA SUB-KINGDOM III 
190 
Norwoodi and in Cryptocrinus melo, the summit is paved by numerous, very 
minute, irregularly arranged pieces. In the genus Pentremites, as observed in 
five or six species, the summit is surmounted by a pyramid formed of five 
elongate, tapering spines, the outermost of which are shorter than the rest. 
This pyramid not only conceals the summit-opening, but also extends over 
the greater part of the spiracles and the anal aperture; but it leaves at 

Fic. 313. 
A, Granatocrinus Norwoodi, O. and 8. Upper face of perfect specimen, with mouth and anus (a) closed by 
plates. Spiracles (sp) separate. B, Orophocrinus stelliformis, O. and S. Upper face with closed peristome and 
exposed anus. Spiracles slit-like. C, Pentremites sulcatus, Roem. Central mouth-opening surrounded by five 
spiracles, the posterior one confluent with the anus. D, Cryptoblastus melo, O. and 8. sp. Upper face with 
central mouth-opening, large anus, and eight spiracles (after Carpenter). 
the median line of each ambulacrum a channel communicating with the 
peristome. | 
The summit in most Blastoids is surrounded by a cycle of five pairs of 
openings ; and between the two posterior ones there is usually interposed 
a single additional aperture. 
The former were regarded by 
Roemer as connected with the 
genital system, and were 
called by him ‘ovarian aper- 
tures”; but they are now 
known as the spiracles. The 
other opening which pierces 
the upper end of the posterior 
deltoid is the anus. 
The form and _ arrange- 
ment of the spiracles is ex- 

Fia. 314. 
A, Pentremites Godoni, Defr. sp. Upper face with ambulacral 
fields in various states of preservation. a, Ambulacrum after the 
removal of lancet- and side-plates ; hydrospires exposed ; b, Lancet- 
plate with upper surface denuded by weathering ; c, Perfectly pre- 
served lancet-plate bordered by side-plates ; d, The same, but with 
transverse markings of lancet-plate obliterated; e, Ambulacrum 
covered with pinnules (after Roemer). B, Phaenoschisma acutum, 
Sow. sp. Upper face, enlarged; a, Ambulacrui after removal of 
the lancet- and side-plates ; hydrospire slits (iy) cutting through 
radials and deltoids ; b,c, Ambulacra in which lancet-plates (1) only 
are preserved; d, e, Ambulacra intact; lancet-plate concealed by 
side-plates (after Etheridge and Carpenter). 
posterior spiracle is considerably larger than the others. 
tremely variable; they may 
be round or slit-like; they 
may consist of ten separate 
openings, or those of the same 
pair may be confluent with one 
another; and the posterior 
pair may be fused both with 
each other and with the anus, 
in which case the fifth or 
Granatocrinus, 
Pentremites, Pentremitidea, ete. (Fig. 314, 4), are examples of the latter case ; 
Granatocrinus having five circular orifices with tube-like projections, while in 
Pentremites and Pentremitidea the four smaller spiracles are divided into two 
compartments by the terminal median ridge of the deltoids. The posterior 
