vin EGHINODERMATA MORPHOLOGY OF SKELETON 



333 



this is surrounded by a closed circle of six radials. These plates, 

 together with two accessory plates (interradials ?), form the whole 

 test of the cup of the attached Caryocrinus, from the point of 

 insertion of the stem to the base of 

 the arms. The anus lies excen- 

 trically on the oral surface, in the 

 (interradial) prolongation of the 

 suture between the two larger infra- 

 basals (cf. Figs. 294, 295). 



Eehinoenerinus, five-rayed (Fig. 

 296). The circle of infrabasals 

 consists of four plates, one large 

 posterior plate and three smaller 

 ones. The larger plate is double 

 (i.e. consists of two fused plates). 

 Outside the circle of infrabasals 

 comes the closed circle of the five FIG- 29C. System of plates of the dorsal 

 basalS, and OUtside this that of the cup of Echmoencrmus armatus, spread out 



(after Forbes). For lettering see p. 317. 



five radials, between which acces- 

 sory pieces are intercalated, the homologies of which cannot be made 

 out. The anus lies posteriorly to the right. In CystoUastus the 

 radials, like the radials or fork-pieces of the Blastoidea, have deep 

 incisions on the oral side for the reception of the ambulacra (cf. Fig. 

 259, A and B, p. 312, and Fig. 295). 



B. The Oral System of Plates. 



In certain Echinodermata (Pelmatozoa and Ophiuroidea) there is a 

 system of plates surrounding the oral (ventral, actinal) pole, and thus 

 diametrically opposite to the apical system. This system develops 

 round the left coelomic vesicle of the larva in a way similar to that in 

 which the apical system develops round the right vesicle. The oral 

 system is, however, much simpler than the apical, and consists of one 

 single circle of five plates (less frequently six, in the six-rayed arrange- 

 ment of the whole system) ; these plates, placed interradially, corre- 

 spond in the oral system with the basal plates of the apical system, 

 and are called oral plates. 



In our considerations of this oral system we again find the best 

 starting-point to be the stalked larva of Antedon (Pentacrinus stage). 

 In a young stage of this larva the oral surface of the calyx appears 

 vaulted over by a roof closed on all sides. The surface of the calyx 

 thus forms the floor, and the vault the roof, of a closed cavity, which 

 is called the oral or tentacular vestibule. At the centre of the floor 

 the oral aperture breaks through, connecting the intestine with the 

 vestibule. The mouth is thus at this stage not connected with the 

 exterior. The fifteen primary tentacles, which rise on the disc of 

 the calyx, also cannot project externally, but are covered over by the 



