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



Such are the distinctive characters of this family, which only Bronn understood in 

 the sense in which we now treat it, including the three genera Dexiobranclisea, Spongio- 

 branchasa and Pneumonoderma. 



Description. — The body of the Pneumonodermatidae is more or less elongated, and 

 rounded at its posterior extremity. The length of the cephalic region and the form of the 

 lateral gill vary in the different genera. 



The foot shows a posterior, generally long lobe, and two rather narrow antero-lateral 

 lobes, united throughout their entire length to the cephalic region, between which 

 posteriorly is a small tubercle. 



The fins, tapering at their free end, are nearly triangular in shape, with bent margins; 

 they are bluntly contracted at the posterior side of the base. 



The colour in the individuals of this family is nearly uniform, being violet-brown. 

 A dorsal glandular spot must be noticed as another character common to the three 

 genera of Pneumonodermatidse ; this also exists in Clionopsis, 1 but it is here smaller 

 and longer (see Spongiobranchwa, PL I. fig. 7). 



The suckers of the buccal cavity are generally inserted on appendages ; three in 

 Dexiobranclisea (two symmetrical and lateral and one median, anterior during the 

 evagination of the front part of the digestive tract), and two (lateral and symmetrical) in 

 Spongiobrancliwa and Pneumonoderma. 



There is on the ventral wall of the buccal cavity in the two last genera, in front of 

 the lateral appendages (during the evagination of the proboscis) a small median papilla. 

 I found this papilla in Spongiobranch&a australis (fig. 2, p. 19), Pneumonoderma 

 violaceum, Pneumonoderma pacificum (PL II. fig. 5), and Pneumononoderma souleyeti 

 (PL II. fig. 6). Boas, who previously noticed this in some specimens of Pneumonoderma, 

 regarded it as the rudiment of the median acetabuliferous appendage of Dexiobranchasa. 2 

 The acetabuliferous appendages, which are inserted on the ventral wall of the 

 buccal cavity, are flattened parallel to the body-wall, and show many different forms, 

 being often short, broad and triangular, or else long, narrow, and with nearly parallel 

 margins. 



In Spongiobranchsea and Pneumonoderma the lateral appendages, which are 

 typically developed, fold backwards when retracted, so that their base is then their 

 most anterior part. They bear suckers only on the side which is turned against the 

 wall of the buccal cavity during retraction. During the extension of the appendages, 

 the two acetabuliferous surfaces are turned towards one another (since the appendages 

 are inserted on the ventro-lateral sides of the cylinder formed by the exserted part of the 

 digestive tract), in order to be applied together, on opposite sides of any prey which may 

 pass in front of the mouth. 



1 Paul Pelseneer, Recherches sur le systeme nerveux <les Pteropodes, Archives dc Biologic, t, vii. p. 109. 



2 Spolia atlantiea, p. 151, note 1. 



