MICROSCOPICAL STUDIES. 19 



contributed so greatly to build up the huge masses of our mountain 

 limestones, and many of our Jurassic beds, were but gigantic 

 Pentacrinoids of structure practically identical with the stalked larvae 

 of Antedon, that seem so like tiny attenuated Pentacrinoids, 



Study XVII — Greseis, a typical Pteropod. 



The struggle for mastery and bare existence gives many 

 unexpected results : we see the huge monsters of ocean, not of 

 true finny lineage, but interlopers from the land ; we see the cousins 

 of our starfishes and sea-urchins, taking on the outward form of 

 burrowing worms ; insects become, in appearance, indistinguishable 

 from sticks and leaves ; birds leave their kingdom of air, and pursue 

 their livelihood amid the waters, seeking prey by diving and swim- 

 ming ; but perhaps stranger than all, the butterflies of ocean, whose 

 winged and shimmering myriads are familiar to voyagers on the high 

 seas, are nowise akin to the gaudy visitants to our flowers; neither 

 have they relationship, as we might excusably guess, with the great 

 group of the Crustaceans. The latter, diverse as the insects in habit, 

 and ready as they, to change form, and to adapt themselves to any 

 new life where there may be a prospect of easier existence, yet put in 

 no claim to the title, and it is reserved to the humbler molluscs — to 

 creatures allied to the slow creeping snail, and lethargic limpet — to 

 furnish representatives charged with the duty of peopling the waves 

 with gay flutterers. Yes, the Pteropods, as the Butterflies of the Sea 

 are called, are undoubted molluscs, closely related on the one hand to 

 such Gastropods as the snail, on the other to the Cephalopods — the 

 Octopus and Cuttlefish. But before discussing their place in Nature, 

 let us examine the anatomy of the typical form, Greseis aeicula 

 (Rang), which is the subject matter of this paper. 



Greseis is the most slender, but not the shortest of Pteropods. 

 The body is lodged in a delicate needle-shaped shell (whence the 

 name aeicula), not f-in. in length. This shell, transparent and 

 colourless, and composed of carbonate of lime, is very gradually 

 tapered and extremely narrow, even at the broader end. The pointed 

 end is closed, while from the other protrude two tiny wing-like fins, 

 the means of locomotion— hence the significance of the term Pteropod 

 or " wing-footed." Coinciding with the form of the shell, the body is 

 greatly elongated, especially that part lodging the central portion of 

 the viscera — the visceral hump or dome, which is spoken of as the 

 upper end of the animal (see last paragraph of this article). The 

 shell is lined and produced by a fold of the body-wall, called the 

 mantle, between which and the body, a large space, the mantle cavity, 



