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



dorsal surface has a more or less marked triangular shape, and is compressed from the 

 sides as to form a more or less flattened lobe ; its broad base is in the direction of the 

 medio-dorsal line of the body, while the contrary takes place in Peniagone lugubris and 

 Peniagone wyvillii. A glance at the figure will give an idea of its form and position, 

 as I have thought them to be. The anterior side of the lobe is more or less curved, 

 and almost vertical, and passes immediately into the anterior downwardly directed part 

 of the dorsal surface. Its posterior side, on the contrary, which inclines obliquely down- 

 wards and backwards, is significantly dilated and considerably depressed along its middle 

 so as to form two margins, of which the right one sends out four flat, obtuse projec- 

 tions, the uppermost of these forms the top of the lobe. The height of the appendage 

 is about 35 mm. Posteriorly, almost at the base of the lobe are to be observed one 

 or several very small processes, the correct number of which i't is difficult to determine. 

 The tentacles, of which but few remain, seem to be almost equally large ; when extended, 

 they are very long, measuring about 22 mm. Their large terminal part recalls the pre- 

 ceding species by having, especially round the edge, some branched retractile processes, 

 of which two on the outer margin are largest. The inner side of the processes, as w T ell as 

 a great part of the ends of the tentacles, carry a number of small, retractile, papilla-like 

 projections. Only the posterior half of the ventral surface is provided with eight or nine 

 pairs of pedicels, the posterior ones attaining a comparatively inconsiderable size. The 

 calcareous deposits (PL XXXIII. fig. 9), which are visible in great abundance within the 

 perisoma, seem to resemble in form those of Peniagone lugubris, though varying generally 

 more than those in shape as well as in size. The largest deposits have the arms measur- 

 ing about 0"1 mm. in length and often considerably arcuated ; sometimes the arms seem 

 to exceed four in number. The processes are generally very long, and vary between two 

 and four ; in the former case the deposits seem to be slightly smaller, and their processes 

 issue from the ends of their more or less elongated central part ; in the latter case 

 one process proceeds from each of the four arms, near their attachment to the central 

 part. The ends of the pedicels as well as of the tentacles contain partly simple or 

 branched more or less arcuated spicula. partly four-armed deposits without processes, and 

 with the arms often unequal and more or less irregularly curved (PI. XXXIII. fig. 8). 

 It has been impossible to subject the calcareous ring to a closer examination, but, judging 

 from what I have seen, it does not differ in any striking manner from the ordinary 

 form in this family. Two polian vesicles, measuring from 10 to 15 mm. are present. 

 The reproductive organ consists of a number of large, thick bundles of caeca, which 

 open into two particularly wide tubes, which communicate with a single, very wide 

 efferent duct narrowing anteriorly. The alimentary canal is retained in its proper 

 position by elastic bands or threads ; a mesenteric membrane only occurs anteriorly, 

 uniting the foremost part of the alimentary canal and the duct of the reproductive 

 organ with the medio-dorsal line of the body- wall. 



