REPORT ON THE PTEROPODA. 
95 
leads on to a stage which is found in the Gymnosomata. Furthermore, there are in 
Xotarchas many structures in which the whole organisation of the Gymnosomata may 
be foreseen. 
The foot is entirely separated from the visceral sac, as in the Gymnosomata. The 
parapodia (lateral margins of the foot) have become greatly developed, but owing to a 
special modification their free borders have fused dorsally, forming around the body a 
large “epipodial” or parapodial sac, open only in front above the neck, so that swimming 
is performed in Notarchus by the parapodia it is true, but in a manner wffiich recalls the 
propulsion of the Cephalopoda, the water contained in the parapodial sac being expelled 
by its contraction. 
On the other hand the palatine roof, armed with hooks, of Notarchus indicates, as wrn 
have seen, the first origin of the hook-sacs of the Gymnosomata, and the lateral gill is 
homologous with that of the Pneumonodermatidse. Finally, the conformation of the 
nervous system is identical in Notarchus (and also in the Dolabclla neapolitana) and the 
Gymnosomata. 
If, then, we assume a form nearly related to Notarchus, in which the free margins of 
the parapodia have not fused ; in which the creepiug foot has become shortened by disuse ; 
in which the small rudiments of mantle and shell seen in Notarchus have entirely 
disappeared ; in which the covering of hooks found on the palatine arch has been 
divided into symmetrical halves located in two depressions of the wall of the digestive 
tract (thus becoming transformed into hook-sacs like those of Dexiobranchsea ) ; in which 
on the ventral wall of the proboscis there have been formed prehensile organs similar to 
the primitive suckers of Dexiobranchsea ; and lastly, in which the gill has been some- 
what simplified in its structure, — we shall have a type very close to the most primitive 
Pneumonodermatidse. 
In the systematic Report on the Gymnosomata (Relations of the Gymnosomata 
to each other 1 ) I have shown how all the living forms of Gymnosomata may be derived 
from this primitive type. We are therefore justified in saying that the Gymnosomata 
are specialised Aplysioidea, adapted to extremely natatory habits, and to an entirely 
pelagic mode of life. 
VII. SUMMARY. 
A. The Pteropoda do not constitute among the Mollusca a class of the same value as 
the Cephalopoda, Gastropoda, Scaphopoda, and Pelecypoda. 
B. The Pteropoda are not primitive Mollusca, but are a derived and recent group. 
C. They have no affinity with the Cephalopoda. 
D. They are Gastropoda in which the adaptation to pelagic life has so modified their 
external characters as to give them an apparent symmetry. 
1 Zool. Chall. Exp., part lviii. pp. 67-69. 
