REPOET OX THE PTEROPODA. 
33 
not seem to me quite accurate, although it indicates correctly the practical result 
of it. The truth is rather that each part (cephalic or visceral) has performed 
a quarter of a rotation around the longitudinal axis, the cephalic portion from 
left to right and the visceral portion in the contrary direction. A study of 
the retractor muscle shows that such has been the case. 
In somewhat advanced larvae of Cavoliniidae this muscle may be seen to be 
inserted at the right side of the shell. In the adult condition it is always 
inserted dorsally close to the middle line, in such a manner that then the muscle 
is entirely dorsal and parallel to the axis of the body, whilst in the larvae it is 
very oblique. According to Boas, of the two subdivisions of the retractor 
muscle, that which passes on the right side of the oesophagus would be a new 
formation. 1 The examination of Fol’s figure above referred to shows that 
it is nothing of the kind, and that this right branch is the more primitive, 
since it passes to the right side of the head and to the two fins, whilst the 
left branch, which passes along the other side of the oesophagus and only 
supplies the left side of the head, is secondary. 
B. The pallial cavity originates on the right side ; 2 in the adults it is quite 
ventral. 
C. The anus is displaced towards the left ; 3 in the adult it is situated quite 
at the left. 
D. The shield (pallial gland), which is quite symmetrical in the adult 
Cavoliniidse, is still asymmetrical and oblique in the older larvae, 4 its right 
side being the more developed, which indicates that it originates on this side 
and is displaced towards the left. 
E. The embryos of the Cavoliniidae, on the appearance of the apex of the 
shell, curve in order to follow the more rapid development of the right side ; 
there is then a tendency towards the sinistral coiling of the Limacinidae, 5 a 
coiling which still appears in the development of the Cymbuliidae, in which 
the uncoiling is not brought about so soon as in the Cavoliniidae. 
3. Facts are observed even in the adults which prove the rotation which has brought 
about the difference between the coiled and the straight Thecosomata — 
A. The dorsal groove of Clio ( Styliola ) subula. This groove is not parallel 
to the axis of the shell, but oblique ; it commences at the left and terminates 
in the middle line dorsally ; it thus describes a quarter of a circle and conse- 
quently indicates all the successive positions of the dorsal side during the 
quarter of a rotation performed by the visceral portion of the body from left 
to right (regarding the animal from the dorsal side). 
1 Spolia atlantica, p. 185, note 3. 
4 Ibid., pi. v. fig. 2, mb. 
(ZOOL. CHALL. EXP. PART LXVI. 1888.) 
3 Ibid., p. 146. 
2 Fol, loc. cit., p. 175. 
5 Ibid., p. 197. 
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