88 
THE VOYAGE OF H.M.S. CHALLENGER. 
As regards the visceral ganglia, they are asymmetrical in the Aplysioidea {Aplysia, 
Notarchus, 1 Doldbella rumphii , 2 Dolabella neapolitana, &c.) as in the Gymnosomata ; the 
right ganglion is larger than the left, and gives origin to three principal nerves (right 
pallial and two visceral nerves), whilst the left ganglion only gives rise to the left pallial 
nerve. 
In the Aplysioidea ( Aplysia , &c.) I have observed the same plenro-pedal anastomoses 
(cervical plexus) as has been above described in all the Gymnosomata. 
The situation of the osphradium is the same both in the Aplysioidea and the 
Gymnosomata — 'between the genital opening and the aperture of the kidney, a little 
ventrally (compare the figure of Clione, PI. V. fig. 5, j, with that of Aplysia published 
by my esteemed teacher Professor E. Ray Lankester 3 ). 
The careful comparison of the Gymnosomata and the Gastropods shows then that the 
former have very close affinities with the Aplysioidea ; that they differ less from them 
than from the Thecosomata; and that, on the other hand, the Aplysioidea differ less from 
the Bulloidea than from the Gymnosomata. 
Y. DO THE PTEROPODA CONSTITUTE A PRIMITIVE OR 
A DERIVED GROUP ? 
The view has often been expressed that the Pteropoda constitute a primitive group 
in the phylum Mollusca. Haeckel 4 in his phylogeny of the Mollusca shows that Pteropods 
are situated at the base of the two groups Cephalopoda and Gastropoda. 
In the same way von Jhering 5 considered that Pteropods are the ancestors of the 
Cephalopods ; but he has since abandoned the idea of the affinity between these two 
groups. Wagner, on the other hand, points to the Pteropoda as the probable source of 
the Cephalopods. 6 Lastly, several zoologists still regard the Pteropods as primitive in 
consequence of the simplicity which is observed in certain parts of their organisation, as 
for example the circulatory apparatus (Roule, 7 &c.). 
To the question at the head of this chapter we have now to reply : — No, the Ptero- 
pods are not primitive Molluscs ; on the other hand, they constitute a derived group 
among the Mollusca. 
In support of this view, arguments may be adduced from — (1) comparative anatomy, 
(2) embryology, (3) palaeontology. 
1. A. The profound asymmetry of the organisation of the Pteropods indicates a group 
1 Ibid., pi. iv. figs. 94, 95. 
2 Amaudrut, Le Systeme nerveux de la Dolabella Rumphii, Bull. Soc. Philom. Paris, ser. 7, t. x. p. 70. 
3 Mollusca, in Encyclopaedia Britannica, 9th ed., vol. xvi, p. 657, fig. 63, m, between Tc and o. 
4 Naturliche Schopfungsgeschichte, ed. 3, p. 475 ; Generelle Morphologie, t. ii. p. cxiii, and pi. vi. 
5 Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, p. 249. 
6 Die Wirbellosen des weissen Meeres, Bd. i. p. 117. 
7 Recherches histologiques sur les Lamellibranches, Journ. Anat. et Phys., 23 e annee, p. 72. 
