CH. viii ECHIXODERMATA MORPHOLOGY OF SKELETON 379 



edge of the calyx between the bases of the arms, and in the genus Actinometra, 

 where they are chiefly developed near the ambulacral furrows, they have occasionally 

 been observed on the lowest pinnuke as well, and even on pinnule in the middle 

 or towards the ends, of the arms. 



In Rhizocrinus, in each interradius of the tegnien calycis there is only one water 

 pore perforating the oral plate. In Hyocrinus the anal oral plate is perforated by 

 two pores ; the other oral plates either have one pore each or else none at all. 

 Further, in this genus, 2 to 7 pores occur in the plates of the interambulacral areas 

 lying between the oral pyramid and the edge of the calyx, except in the posterior 

 interambulacral area, where they are wanting. 



It is impossible to decide with certainty whether the pores which, in certain 

 L'<i-i,i(r<ita (Actinocrinidce, Melocrinida;llhodocrinida'}, occur at the edge of the calyx at 

 the bases of the arms (and correspond with the arms in number) are the homolcgues 

 of these water pores. The same applies to the slit-like pores which are supposed by 

 some writers to perforate the edges of the plates of the ventral sac in the Inadunata 

 fistulata (along the sutures), and to the pores which are found along the brachial 

 farrows in the Inadunata larviformia. These pores may, in some cases, have 

 been connected with hydrospires (see the next two sections). In many Inadunata 

 there is what appears to be a true madreporic plate in the posterior interradius of 

 the tegnien. 



VI Blastoidea. 



One part of the perisomatic skeleton of the Blastoidea has already 

 been described in connection with their apical system. There are 

 five interradial or deltoid plates, which surround the oral region 

 (peristome), and radiate out from it (Fig. 331). These deltoid plates 

 do not form a closed circle, i.e. do not touch one another laterally, 

 but are separated from one another by the proximal portions of the 

 five ambulacra. 



In describing the rest of the perisomatic skeleton, which, apart 

 from the stem, consists exclusively of the skeleton of the ambulacra, 

 it is useful to select a few typical forms. 



(a) The Ambulaeral Skeleton. 



1. Pentremites. Fig. 263, p. 314, shows a representative of 

 this genus in profile, Fig. 331 from the oral side. The five ambulacral 

 regions, or ambulacra, together form a five-leaved rosette round the 

 peristome (Fig. 331, A, B, C, D, E). They are separated from one 

 another by the five (interradial) deltoid plates (3). The larger, distal 

 part of each ambulacrum fits in between the two limbs (10, 10 6 ) of 

 the forked radials. 



On the egg- or pear-shaped body the ambulacra stretch as far as 

 to the equator, or even further, towards the apical pole. 



The skeleton of each ambulacrum, when most complete, consists 

 of the following parts : 



(a) One lancet plate (6). 



(b) One lower lancet plate (12). 



