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



This specimen is distinguished by its obviously globular form, the dorsal surface 

 1 icing extremely convex, the ventral, on the contrary, almost flat. In size it surpasses 

 all hitherto known forms of this family. The tentacles (PL XLIV. fig. 12) are 

 wide, and their flat terminal part is large, about 9 mm. in diameter, discoidal and 

 provided with a number of unbranched, digitiform, retractile processes, of which those 

 round the edge, and especially two on the outside, are large. The pedicels are very large, 

 of great circuit, and constantly consisting of seven pairs, of which the posterior ones 

 are slightly smaller; the last pair, being generally very small, is not placed on the hind- 

 most part of the ventral surface, but on the inside of and a little in front of the next 

 pair, which consequently is the most posterior. The number of the processes and their 

 arrangement seem always to be constant, but their size changes, though not greatly ; in 

 most cases they are of great size, ecpualling in length almost the breadth of the body ; 

 the hindmost pair is always rudimentary. The body-wall of the thinness of paper is 

 soft and transparent, the radial nerve-cords, the ambulacral vessels and cavities being 

 visible through it. The calcareous deposits consist of small spicula curved in the form 

 of a C (PL XXXIV. fig. 8), which are more numerous than the comparatively larger 

 spinose spicula (PL XXXIV. fig. 9), which, thinly scattered, are visible to the naked eye. 

 Those first mentioned are frequently provided with an obvious enlargement in the 

 middle, and taper towards both of their equally curved ends; the largest ones measure 

 about 0'16 mm. in length, but most of them are considerably smaller; their form varies 

 too, as to the degree of curvature. Those C-shaped deposits, which sometimes give off 

 a third arcuated arm issuing from their middle, in which case the C-shaped form 

 evidently vanishes, seem to lie most numerous on the ventral surface. The straight 

 spinose spicula vary in size, the largest measuring in length about - 92 mm., or 

 sometimes more ; some individuals have those spicula more numerous than others, 

 especially on the dorsal surface ; they are not quite straight always, one or another 

 being more or less arcuated. 



The integument possesses besides those calcareous deposits, masses of small cell-like 

 corpuscles containing a brown pigment. Calcareous deposits of almost the same form as 

 those above described are found in the pedicels, tentacles, and processes, the C-curved 

 ones in the pedicels being a little shorter and thicker ; the large sjjicula are often 

 apparently arcuated, sometimes even provided with one or several branches. The ends 

 of the pedicels as well as those of the tentacles contain a number of larger or smaller, 

 thicker, straight, more or less arcuated spicula, which are almost smooth, excepting their 

 obtuse ends, which are rough and spinose ; sometimes those sjiicula bear a larger 

 branch. The transverse muscular layer of the body- wall and the five longitudinal 

 muscular bands are very thin. 



The five pieces of the calcareous ring are very small and far separated one from another ; 

 each piece consists of a short central part, from which run out towards each opposite side 



