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 
eanal. 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. laeacrinus, Mesoblastus, and Acentrotremites have ten 
separate spiracles, and a large, distinct anal aperture. The typical Codasteridae 
(Codaster and Phaenoschisma), 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 
linear to broad petaloid, and are considerably 
complicated in structure (Fig. 315). The 
centre of each ambulacrum is occupied by the 
lancet-plate, « 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 Fig. 315. 
canal, which communicates by means of the _, 4» Pentremites pyriformis, Say. Portion 
: 5 x z of an ambulacrum, exhibiting the lancet- 
ambulacral opening with an oral ring belonging plate (1); median food-groove of the same 
to the water-vascular system. In a number of OF Beet ape nce G). on Ger oF 
forms (Pentremites, Orophocrinus) there is to {;) 2 A™PGho Nome nus Letter. 
be seen a second, smaller, and extremely thin 
plate underlying the median portion of the first; this is called the wnder 
lancet-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 

