cxlviii Introduction. 



numerous, and the madreporic canals may be branched as well 

 as multiple; but in all Holothurioidea the madreporic tubercle 

 or tubercles are contained within the perivisceral cavity, and it is 

 from thence consequently that the fluid for the ambulacral system 

 is drawn. The madreporic canal, when single, is always to be found 

 marking out the inter-radial space of the dorsal bivium by its 

 suspensory lamellar mesentery. The ambulacral feet of the pedate 

 Holothurioidea are very frequently strengthened, as in Echinoidea, 

 by the addition to them of calcareous discs ; they may, however, and 

 especially along the two rays of the dorsal bivium, as also when 

 scattered over the inter-ambulacral areae, be tuberculate and conical, 

 when they are called ' ambulacral papillae.' All Holothurioidea 

 possess an internal skeleton in the shape of a calcareous ring in 

 relation Avith the pharynx, and thus with the water- vascular ring, 

 the nerve-collar, and the annular pseudhaemal plexus. It consists 

 of five radial ossicles, which are either notched, or, as in Synaptidae, 

 pierced for the passage of the water- vessels and nerves, and of a cor- 

 responding niunber of inter-radial pieces. The five radial ossicles are 

 the fixed points to which the five radial muscular bands which give 

 the body of all Holothm'ioidea a pentamerous character, are attached. 

 As the calcareous skeleton, except in a very few cases (Psolus and 

 Ocmis), is merely represented by spiculae scattered in the substance 

 of the corium, the muscular system is more largely developed than 

 in the other classes of this sub-kingdom, the externally-placed cir- 

 cular layer being especially and distinctively prominent. By means of 

 this highly-developed muscular system the Holothurioidea not only 

 obtain the power of moving in the same way as the Vermes, but, 

 so long as its connection with the nerve-system, and that of 

 the several radial factors of their nerve-system with each other are 

 uninjured, they possess also the singular faculty of ejecting their 

 viscera, and in the case of Synapta digitata of dividing their bodies 

 at various points when injured or alarmed. 



The digestive tract is, with the exception of a few Synaptidae, 

 where it takes a straight antero-posterior course, arranged in convo- 

 lutions, as in the Echinoidea. It consists of a pharynx and an intes- 

 tine, between which a small and short muscular stomach is distin- 

 guishable as interposed. When these animals discharge their viscera 

 upon iri-itation, the intestinal tract is always separated about the line 

 of junction of the pharynx and stomach, and immediately posteriorly 



