REPORT ON THE PTEROPODA. 
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of the Pteropods, and as derived from the Heteropods ! On the other hand, Limacina 
explains the relations of the Cavoliniidae, which if the Limacinidae no longer existed 
would never have been understood, in spite of the existence of fossil Limacinidae ; for the 
morphological results obtained from the study of fossil shells are exceedingly small, and 
render those who devote themselves exclusively to this study liable to singular mis- 
conceptions. 
Finally, thanks to the Limacinidae, it will be possible, as we shall shortly see, to trace 
out the affinity of the Thecosomata to the other Molluscs, and to determine their phylo- 
genetic origin. 
Summary on the Thecosomata. 
The Thecosomata possess only one pair of cephalic tentacles. 
The fins embrace the head as far as its dorsal surface. 
The mantle presents on the floor of the pallial cavity a large pallial gland. 
All the Thecosomata, except Gleba and Cymbuliopsis, have a radula, the formula of 
which is 1 : 1 : 1, two jaws placed laterally, and, contrary to the assertions of Gegenbaur, 
a pair of short salivary glands. 
The stomach is a masticatory gizzard with muscular walls, and exhibits four large 
symmetrical masticatory plates (never two, as stated by Huxley, 1 and copied from him 
by Woodward and Tryon), a fifth posterior plate, and generally eight small anterior 
plates. The liver does not pour its secretion into the gizzard, but into the posterior part 
of the stomach, into which it opens by a very small number of apertures. 
At the anus is generally situated an anal gland. 
The copulatory organ is situated on the anterior dorsal part of the head near the 
tentacles. 
The cerebral ganglia are situated at the sides of the oesophagus and united by a long 
supraoesophageal commissure ; the pleural ganglia close beside the cerebral, and seem 
united with them. 
II. GYMNOSOMATA. 
As may be seen from the systematic Report upon this group, 2 I divide these animals 
into five families, one of which (Pneumonodermatidae) includes three genera, whilst the 
others (Clionopsidae, Notobranchaeidae, Clionidae, and Halopsychidae) include only one each. 
Of the family Notobranchaeidae I was not able to obtain a single specimen for 
anatomical investigation, so that my researches refer only to the other divisions. 
1 On the Morphology of the Cephalous Mollusca, p. 42, Phil. Trans., 1853. 
2 Zool. Chall. Exp., part Iviii. p. 11. 
