12 



8 A Guide to the Zoological Collections in the 



shells", (3) the Aplysiidse or Sea-hares, and (4) the 

 Pleurohranchida; or " Umbrella- shells", are illustrated 

 in Cases 167 A — C by spirit specimens, drawings and 

 models, and by a large series of shells. On comparing 

 the shells of these four families, we find a regular degra- 

 dation from the stout operculated spiral shell of Torna- 

 tella to the thin light simply-rolled shell of Bulla, which 

 in life is largely concealed by the side-lobes of the foot ; 

 and thence, through the simple calcareous disk of Um- 

 brella, to the paper-like plate of the Sea-hare, which in 

 life is hidden in a fold of the mantle. 



a. OPISTROBRANCHIA NON-PALLIATA. 



[€it0£0 168<|1— €]. 



In the Slug-like non-palliate or naked-gill forms the 

 mantle, and consequently the shell, are absent in the adult, 

 and the gill-plume, therefore, when present, is exposed. 

 Professor Ray Lankester subdivides the group into three 

 sub-Orders, namely, (i) the Pygohranchia^ in which the 

 gill-plume has the form of a coronet of tentacles round the 

 anus ; (2) the Ceratonota, in which there is no gill-plume, 

 and the back is covered with hollow filaments, or "cerata", 

 which no doubt assist in respiration ; and (3) the Haplo- 

 7norpha, comprising a few simple degraded more or less 

 worm-like forms which are only recognised as Mollusks 

 by the possession of the molluscan odontophore. This 

 section is well represented by models in Cases 168A — C. 

 and by some fine spirit specimens, from the Andama n and 

 Laccadive Reefs,placed above the Case. 



iv. GASTROPODA [ANISOPLEURA] PULMONATA. 



\j^-&&ZQ I— t)iii at §outh cub, anf) ix to .\b at Jlorth t\\ii 



of (!BaUci-ii]. 



This great Order, of which about 10,000 species are 

 known, includes the Snails and the Slugs, the great 



