REPORT ON THE PTEROPODA. 
35 
Cuvienna Cavoliniu Gleba 
Clio Gijmbuliopsis 
Limacina Cymbulia 
Peraclu 
1. We have thus explained the affinities of the Cymbuliidse and Perciclis. We 
may say, then, that the Cymbuliidse arise from Limacinidse resembling Peraclis which 
have lost their calcareous shell towards the end of embryonic development, and the two 
halves of which (cephalic and visceral) have each performed in opposite directions a little 
less than a quarter of the rotation about their antero-posterior axis, and, lastly, which 
have acquired by a subepithelial thickening of the pallial integument a “ cartilaginous 
shell,” which makes up for the loss of the calcareous shell. 
Gleba is more specialised than Cymbulia and Cymbuliopsis, and in its development 
passes through a stage closely resembling Cymbulia, with a short proboscis and long 
appendage to the foot ; this appendage shortens and disappears, whilst the proboscis 
elongates ; further, in passing from Cymbulia to Gleba the cartilaginous shell diminishes 
in importance. 
In the same way Cymbuliopsis is more specialised than Cymbulia, as shown in its 
more prominent proboscis and the disappearance of the pedal appendage. The shell has 
retained more importance than in Gleba. 
2. Limacina is more specialised than Peraclis ; the head has become less distinct, 
and the tentacles have lost their symmetry ; the nervous system is more concentrated, 
the abdominal ganglion, which is quite distinct and separate in Peraclis (PI. I. fig. 12, d ), 
having fused with the “ supra-intestinal ” ganglion (PI. I. fig. 8). The Limacinae have 
become specialised in two different directions — firstly, by elongating the spire of the shell 
in such a manner as to attain the extreme form of Limacina bulimoides. 
This mode of specialisation cannot, however, be very favourable, the most advantageous 
arrangement for a swimming animal being perfect symmetry, which cannot be realised by 
a coiled shell. Hence, specialisation in another direction is brought about — the uncoiling 
of the spire in such a way as to give rise to the straight forms, which, though sym- 
metrical only as regards their external appearance, are thus adapted better for pelagic life. 
Their symmetry is thus secondary, for their primitive asymmetry remains real, complete, 
and profoundly impressed upon all their internal organisation. 
This uncoiling must have acted like that of the caducous embryonic shell of Gleba, a 
large part of which is straight and separated from the spire. 1 This uncoiling must have 
1 Zool. Chall. Exp., part lxv. pi. ii. fig. 17. 
