22 



brown colour, partifularly on the velum. A verv 

 delicate cuticle is also present. In the young Pecten 

 the epidermal cells near the margin of the mantle 

 and on its outer surface are very long compared with those 

 of the epidermis elsewhere, and are evidently active 

 secreting cells of the shell substance. In adult specimens 

 this great difference is not seen. The columnar cells on 

 the free margin of the mantle lobes, especially on the eye 

 stalks (fig. 'j5), have a very peculiar appearance, due either 

 to delicate connecting bridges like the "' prickle cells " 

 or to the walls having processes which interlock ; most 

 probably the former. Lying amongst these epithelial 

 cells are numerous sense cells (" pinselzellen ""j, to be 

 described later in the chapter on the sense organs. 



Underlying the epidermis, there is at the margin of 

 the mantle lobes (hg. 4) a siibstantial connective tissue, 

 consisting of delicate fibres with few scattered nuclei. 

 There are numerous blood spaces in this layer, and the 

 circumpallial artery (tig. 4, A. c.) and the circumpallial 

 nerve (fig. 4, N. c.) pass through it, in close proximity, 

 the blood vessel being situated on the shell side of the 

 nerve. Passing inwards, away from the margins, the 

 mantle lobes become extremely thin, the structure being- 

 more and more trabeculated until, after passing the line 

 of attachment of the pallial muscles, there is practicalh' 

 nothing between the epidermal layer of cells but bridges 

 of fibrous tissue, large spaces being left in which are to 

 be seen numerous blood corpuscles with large nuclei. 



Eamifying in the connective tissue before mentioned, 

 at the margin of the mantle lobes, are the pallial muscles 

 (figs. 1, 3, 4, P. M. v., V. M. c, V. M.). 



The pallial musculature of Pecten is both important 

 and complex, and the edges of the mantle are very well 

 supplied, owing to the energetic part played by the velum 



