REPORT OH THE PTEROPODA. 
79 
respond to the lateral parts of the embryonic pedal disc, and are comparable to the 
whole lateral portion of the foot of the Gastropoda. 
De Blainville and Boas have pointed out that it is the Bulloidea among the Tecti- 
branchiata that the Thecosomata approach the most nearly, and we shall see that this 
view is quite justified. These authors, however, confine themselves to this mere 
statement without attacking the question whether the Thecosomata are descended from 
the Bulloidea or vice versa, and without trying to ascertain by what course the passage 
has been made. 
Further, Boas is unable to point out for which group of the Opisthobranchia the 
Gymnosomata have the greatest affinity. 
We must then enquire what are the special affinities of the Gymnosomata, and 
whether the Pteropocla are a primitive or a derived group as regards the Tectibran cilia ; 
and, further, according to the answer obtained we must endeavour to show for each group 
of Pteropods (Thecosomata and Gymnosomata) to which group of Tectibranchs it is most 
nearly related, and how the passage from the one to the other has been brought about. 
A. Thecosomata. 
If it were necessary to investigate the relationships of the Thecosomata by reference 
only to the organisation of the Cavoliniidse, the task would present great difficulties, for, 
as we have seen, these animals have undergone an anomalous transformation, which 
quite masks the aspect they would otherwise present, and renders them very different 
from animals to which they are very closely related. 
This is the cause which has led to the affinities of the Pteropoda having been for so 
long misunderstood : — the Cavoliniidse have been taken as types of the Pteropoda, and 
as they could not be classed along with other Mollusca, they have been erected into an 
independent group. 
Fortunately the Limacinidse still exist in our seas, and we have been able to show 
that they are the most primitive Thecosomata, whilst the Cavoliniidse have been derived 
from them by a process which we have indicated above. It is then upon the Limacinidse 
and not upon the Cavoliniidse that we must rely in endeavouring to trace out the 
affinities of the Thecosomata. 
Considering for a moment the operculum of the Limacinidse, we see that Actseon, 
one of the Bulloidea, is the only operculate Tectibranch, and that its operculum is precisely 
similar to that of Limacina — elongated, semi-lunar, and with few coils. The reversed 
coiling of its spire arises from the reverse coiling of the animal and of the shell ; Actseon 
is coiled in the direct (right-handed) way, and has an operculum with a sinistral spire ; 
Limacina, which is coiled in a retrograde direction, has an operculum with a dextral spire. 
Mantle . — At the place where the “ shield ” of the Thecosomata is situated, the roof of 
