7G THE VOYAGE OF H.M.S. CHALLENGER. 



iuteqialmar areas of the yentral perisome. They are coutiuuous over the edge of the 

 disk with the perisomatic plates uniting the rays ; and he came to the conchxsion that 

 while many of the ventral plates are perforated by water-pores which lead downwards 

 into the body-cavity, these openings are never found in the iuterradii at the sides of the 

 disk.^ He termed them " anambulacral " to distinguish them from the " ambulacralen 

 Kelchporen fUr Fiisschen ; " ^ and this name has been conveniently extended both to the 

 plates which they pierce, and also to the remaining imperforate plates of the interpalmar 

 areas. Owing to the large size of the oral plates in Hyocrinus, which are themselves 

 pierced by water-pores (PI. Vc. fig. 6, u'p), the number of these anambulacral plates on 

 the disk is very small. But in a large Pentacriniis or Comatula they may be very 

 extensively developed, and the pores are occasionally to be found on the sides of the disk 

 between the rays (Ph XVIT. figs. 6, 10; PI. XXVI. figs. 1, 2; PL XXXIII. fig. 7; 

 PL XXXIV. fig.;2 ; PL XXXIX. fig. 2. PL LVII. figs. 1, 3, 4 ; PL LIX. figs. 2, 4, Q,—v:p ; 

 PL LXIL). 



At the edge of the disk the anambulacral plates of its upper surface pass gradually 

 downwards into the interradials, which are developed in the perisome uniting the rays ; 

 so that in some species both of Comatula and Pentaci'inus the visceral mass is every- 

 where protected by a continuous armour of plates. Many of the fossil Pentacrinidse and 

 also some species of Apiocrinus show signs of the same structure. It is especially well- 

 marked in the Liassic genus Extracrlnus, which had a very large and thickly plated 

 " ventral sac." In fact the disk of these Crinoids seems to have borne stouter plates than 

 that of many of the Palaeozoic Ichthyocrinidas ; and I do not understand how the ventral 

 disk of this famUy, which is described by Wachsmuth' "as composed of a more or less 

 soft or scaly integument, jdelding to motion in the body and arms," can be compared 

 to anything else than the oral surface of a recent Crinoid, with which, however, 

 Wachsmuth says that it " cannot in the remotest degree be homologised.* " 



I have not seen any good disk of Pentacrinus asteria; but, judging from the 

 condition of its peripheral part in the specimen figured by Mullcr, I imagine it to have 

 been covered with a continuous pavement of tolerably large plates. This is also the 

 case in Pentacrmus wyville-tliomsoni (PL XVII. fig. 6). The interpalmar areas are 

 covered with a very closely-fitting pavement of polygonal plates, the largest of which 

 may be pierced by four or five water-j)ores. The anal tube, which is plated almost up to 

 its summit, occupies the greater part of the corresponding interradius ; but the anambu- 

 lacral plates which are between it and the mouth (in the specimen figured) are smaller 

 than elsewhere, and less distinctly defined. In fact they look as if they had fused into 

 two irregularly- shaped plates which abut directly on the peristome. A similar fusion of 

 small plates appears to have taken place on the anal tube of the Mctacrinus nodosus 



' Bail des Pentacrinus, p. 49. • Bau der Echinodernien, p. 63. 



^ Kevrision, pt.i. p. 31. ' Amer. Journ. Sci, vol. xiv. p. 190. 



