10 THE VOYAGE OF H.M.S. CHALLENGER. 



the medio-dorsal line, its extremity being invested by a calcareous network or not, or 

 piercing the body-wall, and communicating with the exterior by one or several pores. 

 The madreporic canal never depends freely into the peritoneal cavity. Ambulacra! 

 vesicles often transformed into large branched or unbranched cavities lying within the 

 perisoma. Calcareous ring incompletely developed, either composed of a continuous, 

 very fragile, network, without any distinct radial and inter-radial pieces, or mostly of five 

 radial spicule-shaped pieces. Eespiratory trees, ciliated cups, and retractor muscles 

 absent. Sexes distinct. 



This order is divided into three families — Elpidiida?, Dehnatidse, and Psychropotidse. 

 The latter differs considerably from the two former in the shape of the body, which approxi- 

 mates often in general appearance to that of certain Aspidochirotse ; the presence of a 

 more or less distinct margin round the body gives it a rather depressed aspect, and the 

 head portion of the representatives of this family is generally considerably flattened — 

 almost discoidal. The completely ventral position of the mouth, the arrangement of the 

 comparatively minute pedicels in a single row round the above-mentioned brim, the pre- 

 sence of a double row of pedicels along the odd ambulacrum, &c, may be considered as 

 characters which give the Psychropotidas the right of being placed side by side with the 

 two other families. Particular attention must be paid to the lateral pedicels, which are 

 more numerous in the Psychropotida?, and either small and retractile, or resembling more 

 or less prominent protuberances, while those in the Elpidiida? and Deimatidse are as a 

 rule large, wide, cylindrical, and non-retractile ; besides, in both of these families the 

 tentacles are very seldom capable of being drawn within the body, and their dorsal pro- 

 cesses, mostly of considerable length, are often symmetrically arranged in pairs. The 

 Elpidiidse differ from the others in having the calcareous ring constantly composed of 

 only five radial spicula, while in the other families it seems to be made up of a fragile 

 net-work. The Deimatidge are distinguished from the Elpidiida? by their dorsal processes 

 being more numerous, generally longer, densely crowded, and arranged in one or several 

 continuous rows along the dorsal ambulacra ; by their generally more elongated body, 

 by the form of the calcareous deposits, by the number of the tentacles, &c. The 

 ElpidiidaB have, with but few exceptions, ten tentacles, and the perisoma is always 

 strengthened by branched or unbranched spicula. 



Family I. Elpidiid.e. 



Body varying greatly in shape from very long, cylindrical, and Synapta-shaped, to 

 rather short, almost globular, or strongly depressed, nearly flat ; tentacles as a rule ten, 

 in a few forms eleven to twelve or twenty ; mouth seldom completely ventral, usually 

 almost terminal, though more or less distinctly turned towards the ventral surface ; the 

 lateral ambulacra of the ventral surface bearing long and wide, cylindrical or conical, 



