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



of the triangular lobe bear fringes at their bases ; these fringes generally are continued 

 on the lateral somatic crests. The median crest of the gill never bears fringes. 



The Posterior Gill is formed of a quadrangular projecting crest, thin, membranous 

 and transparent, which surrounds the posterior end of the body. There arises from 

 each corner of this principal crest, a longitudinal one directed forwards (the two crests 

 arising on the right side, that is, on the side of the lateral gill, are always shorter than 

 the two crests of the left side). The four radiating crests and the two anterior and 

 posterior sides of the quadrangular one have fringes or foldings at their bases. 



The posterior gill of Pneumonoderma is an adaptive one (whereas the lateral gill 

 morphologically corresponds with that of the Tectibranchiate Opisthobranchs, such as 

 Aplysia, for example) ; it is but a modification of a part of the cutaneous envelope, 

 which must assist the lateral gill in its breathing function. The formation of crests, the 

 development of fringes at the base of these, and the subdivision of these fringes, are 

 successive adaptations, favourable to the aeration of the blood by increasing the 

 surface in contact with the water. 



It is difficult to understand how such a good observer as Gegenbaur, 1 followed in this 

 respect by Bronn, 2 thinking to correct Cuvier, van Beneden, and Souleyet, has stated 

 that the posterior gill of Pneumonoderma only shows three radiating crests. Is his 

 observation based on monstrous or rather on specimens deformed from bad preservation ? 



Buccal Appendages. — Two symmetrical appendages, latero-ventrally inserted on the 

 proboscis (PI. II. fig. 5) and having the form of flattened ribbons, more or less wide and 

 long. They bear pedunculated suckers, 3 on one side only, the median one during the 

 extension of the appendages. 



Proboscis shorter than in Spongiobranchsea, and showing, in all (?) the species, 4 the 

 same median abdominal papilla as in Spongiobranchsea australis, placed in front of the 

 acetabuliferous appendages when the proboscis is protruded. 



Radula, never with a median tooth in the adult (this tooth exists in the larvse) ; the 

 number of the lateral teeth never varies much in the different species from four to six. 



Hook-sacs cylindrical and very long; they contain a great many slightly bent hooks, 

 scattered over the whole wall of the sac, the longest at the extremity and the shortest at 

 the base (during the evagination of the sac). 



The gills of this genus have nearly always been insufficiently described (as in the 

 defective descriptions of Gegenbaur and Bronn), and still more often badly figured. In 



1 Untersuchungen iiber Pteropoden, &c, p. 85. 



2 Die Klassen und Ordnungen des Thierreichs, Bd. iii. p. 607. 



3 The structure of these suckers was studied by Nieraiec (Recherches morphologiques sur les ventouses dans le regne 

 animal, Recueil Zool. Suisse, t. ii., 1885), and by me (The Cephalic Appendages of the Pteropoda, Quart. Journ. Micr. Sci., 

 1885). 



4 I found this papilla on Pneumonoderma violaceum, Pneumonoderma pacificum (PI. II. fig. 5) and Pneumonoderma 

 souleyeti (PI. II. fig. 6). 



