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OX THE ANATOMY OF THE GENUS AGAYUS, Mojjtfort. 



By W. B. Randles, B.Sc, etc. 



(From the Zoological Laboratory, Boyal College of Science, London.) 



Read 6th April, 1900. 



PLATE IX. 



This investigation was undertaken at the suggestion of Mr. 0. Collett, 

 who, as a resident in Ceylon, was able to collect and preserve specimens 

 of the animals of nearly all the important varieties and species of 

 Acavus. The genus, though confined entirely to Ceylon, presents us 

 with a number of species, some of which have several distinct varieties. 



It was with a view to the testing of the validity of these species 

 and varieties from an anatomical standpoint that Mr. Collett collected 

 the specimens forwarded to us; he also furnished notes giving a descrip- 

 tion of the habitat and colour of the living animal in each case. 



The animals were preserved in formol, and were in excellent 

 condition, this medium being undoubtedly a better preservative for 

 many of the Mollusca than spirit. 



In order to make a careful comparison of the different species, it 

 was necessary to undertake a detailed examination of the type of 

 the genus, viz. Acavus Immastomus (Linn.); fortunately there were 

 a number of typical examples of this species. 



The anatomy of this type has evidently been examined by various 

 observers — Semper, Pilsbry, Binney, and others ; but there is no 

 account published that is sufficiently detailed to serve as a basis for 

 comparison with the other varieties and species, which one could only 

 expect to vary in minute detail. 



After a careful examination of several of the species of Acavus one 

 is struck by the almost absolute similarity in the general anatomy 

 of all the forms. There are no conspicuous differences in any of the 

 species, so that a description of the anatomy of any one form will 

 apply equally well to all ; any particular variation occurring in an 

 organ will be noted under the description of that structure. 



The figure (PL IX, Fig. 1) illustrating the general anatomy of 

 Acavus was drawn from a specimen of A. phoenix (Pfr.), merely on 

 account of its large size and beautifully extended and preserved 

 condition. It will be as well to give here a list of the species and 

 varieties of Acavus forwarded to us by Mr. Collett, with his notes on 

 the colour of the living animal and its habitat. 



1. Acavus h^mastomus (Linn.). 



(a) Several specimens, the shells of which were variously coloured 

 and banded; they all agreed, however, in the colour of the animal 

 being the same. 



VOL. IV. — OCTOBER, 1900. 8 



