CLASS III 



BLASTOIDEA 



191 



spiracle in the two latter genera is divided by a duplicate ridge into three 

 compartments; of these the middle one enters the inner cavity, and the two 

 outer ones communicate with the hydrospires by means of the hydrospire 

 canal. In Troostocrinus, Schizoblastus, and Cryptoblastus (Fig. 313, D) the 

 posterior spiracles are confluent with the anus, while those of the four regular 

 sides are separated. Elaeacrinus, Mesoblastus, and Acentrotremites have ten 

 separate spiracles, and a large, distinct anal aperture. The typical Codasteridae 

 (Cadaster and Fhaenoschisma), in which the hydrospires are exposed externally, 

 have no spiracles and no hydrospire canal. Orophocrinus (Fig. 313, B) has 

 ten elongate clefts extending along the sides of the ambulacra ; but these are 

 in reality the unclosed portions of the radial sinuses, and correspond to the 

 open hydrospire canals of Pentremites, which are apparent upon the removal of 

 the side-plates. 



The ambulacra are usually depressed below the general level of the calyx, 

 but are sometimes raised above it, or they may be placed in the same plane 

 with it. They vary in form from narrow A n 



linear to broad petaloid, and are considerably 

 complicated in structure (Fig. 315). The 

 centre of each ambulacrum is occupied by the 

 lancet-plate, a long, narrow piece, pointed at 

 both ends, which extends to the full length of 

 the fields. Its proximal end is inserted be- 

 tween the deltoids, and takes part in the lip 

 around the summit-opening. The upper sur- 

 face of the plate is excavated along the median 

 line, and forms an open, well-defined groove, 

 which conducts to the mouth, and in all prob- 

 ability represents the food-groove. The in- 

 terior of the plate is traversed by an axial 

 canal, which communicates by means of the 

 ambulacral opening with an oral ring belonging plate (0 ; median food-groove of the same 



,, T r (a) ; side-plates (s) ; outer side-plates (e) ; 



tO the Water- VaSCUlar System. In a number OI and marginal pores (p). 5/j (after E and 



forms (Pentremites, Orophocrinus) there is to ^SffSSSSSS^^ Letter ' 

 be seen a second, smaller, and extremely thin 



plate underlying the median portion of the first; this is called the under 

 hi mrf -plate,. 



The lancet-plate rarely occupies the full width of the ambulacral field, and 

 the spaces between its lateral edges and the sides of the radial sinus are either 

 wholly or partially covered by a row of small, horizontally elongated side-plates 

 (" pore-plates" of Roemer). In Pentremites, Orophocrinus, and other genera, an 

 additional series of still smaller pieces, called the outer side-plates (" supple- 

 mentary pore-plates" of Roemer), are placed between the side-plates and the 

 walls of the radial sinus. Pentremites and Cryptoschisma have the entire upper 

 surface of the lancet-plate exposed to view, and the side-plates are situated 

 alongside of it in the same plane. But in other forms the lancet-plate is 

 wholly, or to a very large extent, concealed by the side-plates (Fig. 315, B), 

 so that as a rule only a small space along the food-groove is' visible. The 

 sutures between the side-plates are indicated by shallow, horizontal grooves, 

 which are continued as superficial markings over both halves of the lancet- 

 plate as far as the median ambulacral groove. These crenulations, it should 



FIG. 315. 



A, Pentremites pyriformis, Say. Portion 

 of an ambulacrum, exhibiting the lancet- 



