REPORT ON THE PTEROPODA. 
3 
So too in regard to the habits of the Thecosomata. Like the Giymnosomata they are 
pelagic Molluscs, which descend to a certain depth to avoid bright light, and reascend 
when the light is feeble or absent, and when the sea is calm. They feed mainly on 
Protozoa (Radiolaria, Foraminifera, Infusoria) or on lower Algae, while the Gymnosomata 
prey upon decidedly higher animals. This difference of diet is the condition effecting 
the notable diversity in the structure of the alimentary system, and especially of the 
buccal and stomachic armature. 
The Thecosomata of the Challenger Expedition. 
The Thecosomatous Pteropods collected on the Challenger Expedition, which form 
the material bases of the present Report, comprise two distinct series : — 
1. The Thecosomata captured alive in the tow-nets, and preserved in alcohol or in 
microscopic preparations mounted in Canada balsam or in glycerine. These were entrusted 
to me towards the end of 1885 for use in my Report on the anatomy of the group. 
2. The dry shells from deep-sea deposits, the importance of which will be discussed 
in the special report on the sea-bottom. This collection was selected from the deposits 
in the Challenger Office and also by Mr. Alfred E. Craven, who at one time proposed to 
write the Conchological Report on this group. It passed into my hands in the beginning 
of 1887, when a large number of the specimens had been already assorted. 
I. The Thecosomata taken alive were gathered from seventy stations, and include 
twenty-eight species representing all the known genera. Among these species there 
is no new form, though a certain number have been hitherto insufficiently known or 
only once recorded. 
The following table indicates the distribution of these species among the different 
genera. 
Limacina, 
Peraclis, 
Clio , . 
Cuvierina, 
Cavolinia, 
Cymbulia, 
Gleba, 
that is, among 7 genera, . . . • • • .28 species. 
II. The Thecosomata dredged from the deposits of the deep sea occur in all 
those known by the title “ Pteropod ooze,” and also in others. I have received the 
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10 
1 
8 
1 
1 
