76 
THE VOYAGE OF H.M.S. CHALLENGER. 
In conclusion, as has been said by Fol, 1 the embryonic characters are not sufficient to 
justify the separation of the Pteropoda from the Gastropoda. 
We see then that the Pteropoda possess the principal general characters of the 
Gastropoda and especially the visceral asymmetry which results from the unilateral 
development of the visceral organs and characterises the specialised, that is to say the 
most numerous, Gastropoda. 
But if we seek out from among the Gastropoda those forms with which the Pteropoda 
have the greatest affinity, we find the common characters still more numerous. 
1. In the Pteropoda, as in the Tectibranchia, 2 a partition, or species of diaphragm, 
divides the body-cavity into a posterior visceral and anterior or cephalic portion, this latter 
enclosing the buccal mass, the central nervous system, and the copulatory organ. 
2. The salivary glands of the Pteropoda, like those of the Tectibranchia ( e.g ., 
Bulloidea and Aplysioidea) do not exhibit a duct differentiated off from the secretory 
portion. 
3. The stomach in the Thecosomata at all ages and in the larval Gymnosomata has 
masticatory plates, as in the great majority of the Tectibranchia. The adult Gymnoso- 
mata in consequence of their diet have an unarmed stomach like that of the carnivorous 
Tectibranchia {e.g., Doridium)? 
4. The liver of the Pteropoda is disposed like that of the Tectibranchia, the 
Gymnosomata resembling Gastropteron, the Thecosomata the Bullidse in this respect. 
5. The generative gland of the Pteropoda is hermaphrodite like that of the Tecti- 
branchia ; as in the Aplysioidea and Bulloidea it possesses a single undivided efferent 
duct with a single orifice. As in these two the genital aperture is connected by a 
spermatic groove with the copulatory organ which is situated in the head. 
6. The pedal ganglia both of the Thecosomatous and Gymnosomatous Pteropoda have 
two commissures like those of the above-mentioned Tectibranchia, Aplysioidea, and 
Bulloidea (the second commissure figured in Cuvierina and also seen in Cavolinia has 
probably escaped notice in the other genera on account of its small size). 
The Pteropoda are thus clearly separated from all the other classes of Mollusca, 
whilst they present all the characters of the “ typical ” (asymmetrical but not primitive) 
Gastropoda. 
Further, among the Gastropoda their whole organisation (hermaphroditism and the 
structure of the nervous system) separates them from the Streptoneura (Prosobranchia 
1 Fol, ibid., pp. 197, 198. 
2 Vayssiere, Recherches anatomiques sur la famille des Bullides, Ann. d. Sci. Nat. (Zool.), s4r. 6, t. ix. p. 78. 
3 Vayssifere, Recherches zoologiques et anatomiques sur les Mollusques Opistobranches du Golfe de Marseille, 
i Tectibranches, Ann. Mus. Marseille, t. ii. p. 44. 
