38 
THE VOYAGE OE H.M.S. CHALLENGER 
Family I. Pneumonodermatida 
The three genera which make up this family resemble each other very closely in their 
general internal organisation, since their principal zoological differential characters are 
external, taken from the buccal acetabuliferous appendages and the gills. 1 I think, 
therefore, that the best course wfill be to examine these three genera simultaneously 
rather than separately, in order to avoid numerous repetitions ; I shall mention in each 
case, however, the points in which I have observed differences between Dexiobranchsea, 
Spongiobranchasa, and Pneumonoderma. 
The Head in the Pneumonodermatidse is somewhat elongated. It presents anteriorly 
the buccal opening, dorso-ventral in direction, and the two anterior or labial tentacles, 
situated on either side of it and dorsally rather than ventrally. These tentacles are more 
or less elongated ; those of Spongiobranchasa are more elongated than in others. They 
are not the seat of a special sense but of general sensibility; their distal extremity 
encloses elongated nervous cells. 
On the dorsal surface of the cephalic region, near the middle of its length, is a pair of 
posterior or nuchal tentacles, which are quite symmetrical but only slightly prominent. 
Each of them receives two cerebral nerves, each of which is swollen at its distal extremity 
within the tentacle, an arrangement which perhaps led Souleyet to believe that they are 
bifid. 2 
These tentacles are difficult to discover when retracted in preserved specimens, but 
are readily found from within owing to the presence of the nerves. We shall shortly 
see that these are the optic and olfactory nerves, and their distal enlargements are the 
rudimentary eye and the olfactory ganglion or rhinophore. 
The Foot is similarly shaped in all three genera, as has been already described in the 
systematic portion of this Report. The plicated tubercle at the base of the posterior 
lobe is glandular in function. All the ventral surface of the foot is ciliated. 
The visceral envelope, continuous with that of the head, has several kinds of sparse 
unicellular glands scattered all over it. The most considerable are aggregated in the 
middle line on the dorsal surface, where they form a depression known as the dorsal 
patch. A transverse section through this (PI. IV. fig. 7 ) shows the presence of two kinds 
of glands — (l) the peripheral or lateral glands (6), which are very large cells; (2) the 
median glands (c), small in size and whose secretion is of a bright colour. 
The Digestive Tract . — The anterior portion from the mouth, as far as the buccal mass, 
constitutes an evaginable proboscis of the acrembolic type. This has been figured in three 
genera in the systematic part of this Report ; it is least developed in Dexiobranchsea. 
On the anterior part of the retracted proboscis (posterior part when it is evaginated) are 
1 Compare the systematic Report on the Gymnosomata, Zool. Chall. Exp., part lviii. pp. 1 1-32. 
2 Voyage de la Bonite, Zoologie, t. ii. p. 256. 
