312 



PHYLUM ECHINODERMATA. 



groove is formed by a median plate called the lancet plate (Fig. 225, 2, L) 

 and on each side by lateral plates (sp). The lancet-plate contains 

 a canal (Fig. 225) which in the neighbourhood of the mouth opens 

 into the cavity of the calyx. The ambulacra! groove possesses cover- 

 ing plates (c.p) which can only be discerned in a few specimens and 

 which are continued over the mouth. Crossing, at right angles, the 

 sutures between the radials and deltoids, are some deep slits (Fig. 225 h) 

 which lead into pouches projecting into the calyx-cavity (Fig. 225, 2). 

 These pouches are the hydros pires ; they are disposed parallel to the 

 ambulacral grooves (Fig. 225). Pinnules or brachioles (br) are attached 

 to the side plates and furrows pass from the ambulacral groove across the 

 exposed part of the lancet plate on to them. When preserved, they are 

 usually folded over the grooves. There is no evidence for the existence 

 of tube-feet. The anus is in an interradius at the junction of a deltoid 

 with two radials (As). It is not in the interradius of the small basal. 

 The arrangement of the hydrospires described above is that found in 



Cadaster , in other Blastoids it is 

 slightly different. They are usually 

 arranged more compactly and their 

 openings become covered over by ex- 

 tensions outwards of the lancet plate 

 and the side plates of the ambulacral 

 groove (Fig. 226). A canal the hydro- 

 spire canal is thus formed. It opens 

 to the exterior by a series of apertures 

 between the side plates on each side of 

 the lancet plate, called the hydros pire- 

 pores, and by a larger opening near the 

 mouth between the deltoid, the proximal 

 side-plate and the lancet plate ; these 

 openings are called the spiracles. They 

 are ten in number, but frequently they 

 become united in pairs, one joining 

 with that of the adjacent radius, so 

 The spiracles adjoining the anus are often 



FIG. 226. Section across a radius of 

 Pentremites (after Bather), br pin- 

 nule (brachiole) ; c.p covering plate ; 

 o.s. p outer side-plate ; .R radial; s.l 

 sub-lancet plate (in many genera a 

 sub-lancet plate is found beneath 

 the lancet plate) ; s.p side plate. 

 The hydrospires here open into a 

 canal beneath the side plates and 

 lancet plate called the hydrospire 

 canal. 



that there are five only, 

 confluent with it. 



In some genera (Astrocrinus and Eleutherocrinus] one of the ambulacra, 

 is different from the rest. The meaning of the hydrospires is quite un- 

 known. They have been compared to the genital bursae of Ophiurids 

 and supposed to have been respiratory in function. They have also been 

 compared with the canals of the pore-rhombs of Cystids. Their relation 

 to the sutures between the deltoids and radials which is so clearly seen in 

 Cadaster no doubt suggests this comparison, and if there is anything in the 

 suggestion on p^.308 that the canals of the pore-rhombs are due to foldings 

 of the stereom, there may be something in it. 



In the general form of their body the Blastoids present a certain 

 resemblance to Echinoids, but they differ from these in the fact that they 

 are usually stalked, in the presence of pinnules, and in the composition 

 of the calyx. The principal genera are : Pentremites Say (Fig. 227), 

 Devonian and Carboniferous. Mesoblastus E. and C., Carboniferous. 

 Troostocrinus Shum., Silurian. Tricoelocrinus M. and W., Carboniferous 

 (Fig. 227). Nucleocrinus Conrad (Elaeacrinus Roemer), Devonian. Orbi- 

 tremites Austin (Qranatocrinus Hall), Carboniferous (Fig. 227). Heterc- 



