308 DR. H. LYSTER JAMESON ON 



continuous with the bases of the epidermal cells, are attached to 

 the ends of the muscle-fibres (muse), which may be produced out 

 into tails. Tullberg recognised that such a junction was present. 

 In Margaritifera vulgaris the attachment epithelium-cells may 

 have one or several tails, probably also of connective-tissue 

 character, passing over into as many muscle-fibres. 



Distally these epidermal cells broaden out somewhat, ending 

 ill a clean-cut surface, which may be represented in section by 

 a clear zone forming a slightly acute angle with the sides. The 

 nuclei, which ai^e oval and about 3-4 ^ long, are situated in 

 the middle of their length. 



It is possible to make out, in some cases, a striation of these 

 cells in the direction of their long axes. Occasionally the distal 

 surface is laised into processes and papillse, but this may well 

 be a result of imperfect fixation of the tissues. 



These cells stand out as stiff", independent, almost bristle-like 

 entities, and ai-e probably hard and tendinous in character. 

 They are frequently preserved in old preparations in which 

 all traces of the structure of the ordinary epithelia have 

 disappeared. In some cases they seem to have been drawn out 

 in the fixing process ; thus the longest cell shown in PI. XXXIII. 

 fig. 4 a measured 26 /i. It seems possible that in the shrinkage 

 consequent upon fixation the majority of the elements here 

 had broken away from the shell, but that this particular cell 

 had remained attached and was consequently fixed in a state 

 of extension. This figure shows that the connective-tissue 

 elements extend up between the bases of these cells. 



This epidermis is very closely adherent to the specialised shell- 

 layer (hypostracum) to which it is attached, and the connection 

 seems to be between the cells and the organic basis of the shell. 

 Thus, in decalcifying a piece of the shell of Mytilus with the 

 adductor muscle attached, the hypostracum remained adherent 

 to the epithelium and tore away from the rest of the shell 

 (PI. XXXIV. fig. 5). 



(11) Shell-Secretion, 



I will not attempt to survey the writings of previous inves- 

 tigators on this subject. This has been ably done by Stempell 

 (45), whose review contains a full and lucid discussion of the 

 question. 



The general trend of opinion now seems to favour the theory 

 dating back to Reaumer, 1709 (31), and held by Tullberg, 

 Ehrenbaum, Moynier de Villepoix, and the majority of recent 

 French and German investigators, that the shell (except the 

 outermost layer of the periosti'acum and the hypostracum) is 

 formed from a fluid secretion, rather than the theory specially 

 associated with Huxley's name (24) that the shell is derived 

 from a succession of fully developed skins or cuticles, shed as 

 membranes hj the underlying epidermis. 



