REPORT ON THE PTEROPODA. 
31 
is ventral in the Limacinidm (the Cymbuliidse, in which the calcareous shell falls off at 
the end of embryonic development, have of course no retractor muscle.) 
5. The fifth large gastric plate, which is dorsal in the Limacinidse, is ventral in all 
the straight forms. 
6. The aperture of the bile-duct, which is at the right in all the straight Thecosomata, 
is at the left in the Limacinidse. 
7. The intestine, which ends at the right of the pallial cavity in the Limacinidse, 
ends at the left of the same cavity in the straight forms. 
8. The genital duct, which arises from the dorsal edge of the gland in the straight 
Thecosomata, springs from the ventral side in the Limacinidse. 
9. The shell, in those straight forms which have any curvature, is bent towards the 
dorsal surface, whilst in the Limacinidse the shell is twisted ventrally. 
Thus on comparing these two groups it appears that, taking one of them as a 
standard, the anterior part of the other has rotated upon the posterior part through a 
half revolution (180 degrees) upon the longitudinal axis or vice versd. 
Now since we have seen that each of the two families of straight Thecosomata has 
clear affinities with the Limacinidse, without their exhibiting any relationship to each 
other, we have still to show which of the three families is the most primitive, and what 
has been the line of their descent. 
I. As regards the Cymbuliidse and Limacinidse, there can be no doubt that the 
Limacinidse are the more primitive and have given origin to the Cymbuliidse. The 
development of the latter furnishes in support of this view excellent arguments which 
have not hitherto been sufficiently appreciated : — 
1. The embryonic shell of the Cymbuliidse is a coiled one, whence these forms are 
classed by Fol 1 among his “ Campyloconques ” in opposition to the Cavoliniidse, 
which he terms “ Orthoconques ” ; but neither Krohn 2 nor Fol 3 state whether the 
shell is dextral or sinistral, and it is impossible to ascertain this from the figures 
given by Krohn. 4 I have seen many embryonic shells both of Cymbulia and 
Gleba, 5 and all were sinistral like the shells of the Limacinidse. 
2. In the older embryos, which yet bear shells, the pallial cavity is dorsal G and the 
anus to the right, 7 just as in the coiled Pteropods. 
3. These same embryos then carry an operculum on the foot, which is multispiral as 
has already been pointed out by Krohn, 8 who, however, did not notice the 
direction of the spire. The examination of numerous specimens with the oper- 
culum in situ enables me to affirm that its spire is always sinistral. Now this 
1 Sur le developpement des Pt^ropodes, Arch. d. Zool. Exper., s&r. 1, t. iv. p. 178. 
2 Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropodeii. 
3 Loc. cit. 
6 Zool. Chall. Exp., part lxv. pi. ii. figs. 14, 17. 
7 Krohn, loc. cit., p. 20. 
4 Loc. cit., pi. i. figs. 12, 14. 
6 Krohn, loc. cit., p. 19. 
8 Krohn, loc. cit., pi. i. fig. 15. 
