viii ECHINODERMATA MORPHOLOGY OF SKELETON 331 



the third consists of perisomatic plates, which, in all probability, 

 correspond with the primary interradii of the Crinoids. 



The first circle at the apex is that of the (interradial) basal plates. 

 There are always three of these, one smaller and two larger of equal 

 size, as also occurs in the Crinoids. The monocyclic base of the 

 Blastoidea is thus symmetrical. But the line of symmetry (the so- 

 called dorsal axis), which passes between the two larger plates and 

 through the small unpaired plate, does not coincide with the 

 symmetrical (ventral) axis of the body, which passes through the 

 mouth and the anus, the latter lying in the posterior interradius on 

 the oral surface. The smaller unpaired basal plate lies in the left 

 anterior interradius. If we imagine the two larger basal plates cut 

 into two similar parts by radial lines of division, we obtain the five 

 equal-sized, strictly radially arranged, and interradially placed basals 

 of most other Echinoderms. The uppermost ossicle of the stem 

 is inserted at the point where the three basals of the Blastoids meet. 



The circle of the basals is immediately surrounded by that of the 

 radials. The typical number of five is always retained in these, 

 which, in regular Blastoids, are strictly radiate in 

 their arrangement. These are called fork-pieces, 

 because each of them is produced upwards, i.e. 

 orally, in the shape of a tuning-fork, the two 

 limbs holding between them the distal end of an *i 

 ambulacrum. The radials form a closed circle, 

 their lateral edges being contiguous. 



The third circle of plates is in immediate 

 contact with the radials, and surrounds the peri- 

 stome. It consists of five interradial plates, 

 which, in regular Blastoids, are strictly radial ; 

 these are the interradials or deltoid plates. CMX 



These plates do not form a closed circle, as they & & 



are separated from one another by the five FlG 2 o 3 . _ Eieuthero- 

 ambulacra. The apical edges of each deltoid crinus Cassedayi, from 

 plate rest on the oral edges of the contiguous ^d e^^m^^^entert 

 forks of two consecutive radials or fork pieces. aa . bbj A * pa . SS ing through 

 The relative sizes of the basals, radials, and inter- the mouth and the anus ; 

 radials of the Blastoids vary greatly (cf. figures). ^^0^ 

 One of the five interradials, which is distinguished an> anal side, 

 as the posterior, is perforated by the anus. 



In the irregular Blastoids (Fig. 293), which are without stems, 

 all the plates of the regular forms are found, but are, naturally, 

 irregularly developed. The radial which supports the modified ambu- 

 lacrum is smaller than the other radials and differently shaped. It 

 appears shifted quite on to the oral surface. At the same time, the 

 pair of basals (y and z) which flank this radial are much prolonged 

 orally as narrow plates. 



It cannot at present be decided whether there are skeletal pieces 



