CRINOIDEA. 281 



The ambulacral grooves themselves are often protected by 

 covering plates which arise at their sides and project over them : 

 these could apparently be erected and depressed and are alter- 

 nate on the two sides of the groove (Fig. 193, A, D, 195). The 

 grooves are sometimes unprotected and open. In the Camerata 

 it frequently happens that the interambulacral plates of the 

 calyx-cover project over and cover up the ambulacral grooves 

 (and their covering plates) which are thus converted into canals 

 open at the edge of the disc where the arms are given off (Fig. 

 193, A}. The mouth, being covered by the firmly united orals, 

 communicates with the exterior, in such cases, only through 

 these canals which branch as often as the arm branches before 

 leaving the calyx. There is, however, as has already been stated, 

 considerable variation amongst the Camerata with regard to 

 this character. 



Covering plates are found in some living forms, e.g. Hyocrinus, 

 Holopus, Rhizocrinidae, some species of Antedon. 



It appears, therefore, if the present accounts of Palaeontologists are to 

 be trusted, that the mouth of the Camerata is, like that of the Larvi- 

 formia, subtegminal. In both these groups, therefore, the only com- 

 munication of the mouth with the exterior is by the ambulacral 

 grooves which issue through openings between the bases of the central 

 tegminal plates. In some Fistulata the grooves are open the whole way, 

 and the mouth is not subtegminal ; in others some of the central 

 covering plates are enlarged and fixed, so that the grooves open im- 

 mediately beyond them and the mouth is subtegminal. In some of the 

 Camerata the ambulacral grooves open to the exterior only at or near the 

 edge of the calyx (Fig. 193, A), the first part of their course- being covered 

 up by the encroachment of the interambulacral plates. 



Further particulars as to the structure of the calyx-cover. In the Fistxi- 

 lata the calyx-cover is usually flat, except in the posterior interradius in 

 which it is prolonged into an enormous sac-like process which no doubt 

 contained a considerable part of the animal's viscera and had the anus at 

 or near its apex. Both process and interambulacra generally are firmly 

 plated. 



In the Camerata the whole calyx-cover is symmetrically prolonged into 

 a vault-like process (Fig. 202), at the end of which was the mouth covered 

 up by the rather indistinct orals, and on the posterior side of which was 

 the anus. The vault is firmly plated and its covering consists of the 

 interambulacral plates and their extensions over the ambulacral grooves. 



In the Flexibilia the calyx-cover is only known in Taxocrinus. It was 

 flat and flexible with numerous loosely arranged interambulacral plates 

 (Fig. 203). The ambulacral grooves are exposed and the orals are dis- 

 tinct and surround the freely open mouth. 



In Holopus and Hyocrinus the structure of the calyx-cover has been 

 sufficiently explained (p. 280). In most other recent forms the calyx-cover 

 is membranous and has only loosely connected plates in the interambula- 



