BLASTOIDEA. 



301 



certain that they represent a series of five recumbent arms 

 essentially similar to the arms of the Crinoids, but im- 

 bedded in the calycine integument, and modified for their 

 connection with the respiratory tubes. Each pseud-ambu- 



Fig. 178. Blastoidea. A, Pentremites pyriformis, side view. B, Base of the same, c, 

 Summit of Pentremites conoideus : b, 5, Basals ; d, d, Radials ; p, p, Pseud-ambulacra, 

 c, Shows the central pentagonal aperture, surrounded by the five openings at the summit 

 of the deltoid plates. Carboniferous. 



lacruni is grooved by a median furrow, or " ambulacral 

 groove," is deeply striated transversely, and, when perfect, 

 carried on each side a series of short jointed filaments, 

 corresponding with the " pinnule " of the arms of the 

 Crinoids, 



There are two other points as to the calyx of the 

 Blastoidea which require notice namely, the respiratory 

 tubes and the apical apertures. As to the first of these, we 

 have seen that, in the majority of the Cystideans, the test is 

 more or less freely pierced by pores, which admit the sea- 

 water to a series of delicate internal tubes, or sacs, and 

 which may thus reasonably be supposed to have exercised 

 a respiratory function. In some Cystoids these pores and 



