1921.] N. Annandale & R. B. S. Seweli, : Vivipara. 255 



It is, however, in Lecythoconcha, in which also the mantle is 

 thick, that the muscles are the most powerful among the forms 

 examined. The external retractor and its branches are l)oth very 

 coarse, but the latter are not numerous and the muscular network 

 is not well developed. The sphincter, however, is both thick and 

 complex, consisting of several strands which run obliquely in the 

 midst of the shell-glands. Theii position in reference to the edge 

 of the mantle differs in different states of expansion and retraction 

 (figs. 3, 4, pi. iii). 



Nerves. — I have not attempted to work out the nervous sys- 

 tem of the marginal region in detail and have not observed any 

 external sensory organs. The whole of the roof of the branchial 

 chamber is supplied by nerves arising from the parietal ganglia 

 {Seweli, p. 240). In the marginal region a fairly stout transverse 

 nerve can be readily distinguished, pursuing an irregular course 

 above the supramarginal ridge, some parts of it being much nearer 

 the margin than others. From it finer nerves run down at irregular 

 intervals am.ong the shell-glands. Their position is not definitely 

 correlated with that of the marginal processes (fig. 12 B, p. 248). 



V.ASCULAR System. — The marginal region of the mantle is 

 highly vascular in all species of Viviparidae examined. Definite 

 blood-vessels can he seen entering it, but for the most part the 

 blood is contained in irregular sinuses without definite walls. 

 These reach their maximum development in the primary mar- 

 ginal processes of Vivipara oxytropis (pi. iii, fig. 5), in which the 

 connective tissue has a strictly cavernous structure. A vascular 

 system of this type cannot be investigated in detail without 

 careful injection. This method I have not attempted to adopt as 

 it is quite sufficient for my purpose to know that the processes, 

 and indeed the whole of the edge of the mantle, are erectile rather 

 than muscular, though their erectility is doubtless correlated with 

 the action of the muscles of the roof of the branchial chamber. 



Sheli -GLANDS. — A most important part of this investigation 

 refers to the structure, position and function of the glands that 

 secrete the substance of. the external layers ' of the shell and their 

 relation to the external structure of the marginal region of the 

 mantle and the ornamentation of the shell. The main facts about 

 these glands have long been known and certain important points 

 were made clear b}' I/cydig,"^ Mer and Longe ^ and Moynier de Ville- 

 poix,* but I have failed to find in zoological literature anj' discus- 

 sion of their comparative anatomy and functions in any one family 

 of Gasteropods. As my own observations are in general agreement 

 with those of the authors cited I will give an 'account of what I 

 have myself seen without further historical discussion. 



' I do not propose to deal with those that secrete the internal nacreous layer. 



i Zeits. f. Wiss. Zool. II, p. 123 (1850). 



3 Comp^ Rendiis XC, p. 882 (1880). 



* Comp. Rendits CXIII, p. ,^,17 (1891) and C\\, p. 512 (1895). 



