22 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



confinement and fertilizes the ova, which are then ejected in long 

 gelatinous cords that float hither and thither at the mercy of the 

 waves till hatching takes place. 



Adult Pteropods all progress by jerky flappings of the wings. 

 Agassiz says they can remain suspended in the water for hours, 

 simply by spreading these wings, and then suddenly drop to the 

 bottom by folding them. They are also said to creep about by means 

 of these same appendages. 



The group of Pteropoda is not large, and its members fall 

 naturally into two well marked divisions — those like Greseis, with a 

 well developed shell, form the order Thecosomata ; those naked and 

 without shell, the Gymnosomata. Few are ever seen near land, they 

 prefer the high seas, and are spread under all latitudes, little more 

 plentiful in the Tropics than in the Northern regions of Baffms Bay 

 and Davis Strait and the Polar Sea generally — where indeed the 

 multitudes of two species, Clione borealis and Limacina arctica, 

 form a substantial item in the dietary of the whale. 



Id considering the place in nature of these animals, the possession 

 of an odontophore (lingual ribbon or radula) at once discovers their 

 close relationship to the Gastropods and to the Cephalopods, and with 

 these and the little group of Scaphopods (Dentalium), form the com- 

 pact branch, Glossophora^" tongue-bearers"), of the phylum Mollusca. 

 (a) In the arrangement of the genital system, the Pteropods are 

 extremely like many forms of hermaphrodite Gastropods ; the snail 

 and the sea-slug (Aplysia) for example, agreeing closely in all the 

 larger details, while in this, they differ markedly from the Cephalopoda, 

 where the sexes are always separate. (6) Outwardly usually bilaterally 

 symmetric like the Cephalopods, Pteropods are all fundamentally 

 asymmetric, and here again approach to the most usual Gastropod like- 

 ness, for as in the latter, both the anus and the sexual organs are lateral 

 and asymmetric, the one in Greseis being turned to the right, the others 

 to the left, (c) A third link with the Gastropods is found in the 

 possession by certain genera (Spinalis) of an operculum, (d) On 

 the other hand, Pteropods of the shell-less group, have processes 

 developed from the " head," of arm-like form : in some cases even 

 bearing suckers — a wonderfully close approach in appearance to the 

 familiar arms of the Octopus and the Cuttlefishes. So close, indeed, 

 is this resemblance, that Prof. Ray Lankester has not hesitated to 

 class both Pteropods and Cuttlefish in one all-embracing division, the 

 Cephalopoda, forging a new term, Siphonopoda, for the diverse com- 

 pany of the Octopus, Nautilus and Cuttles. Such considerations as 

 a b and c make against this view, and it is significant that the nerve 

 supply to these head-arms has different origin in the two divisions, 



