VII 



MOLLUSGA~THE ALIMENTARY CANAL 



197 



which serves as reservoir and duct for the pigment, 

 in the gland, passes through an 

 aperture in its wall into this 

 reservoir. The cavity of the 

 gland is traversed by numerous 

 perforated and richly vascular- 

 ised lamellas of connective 

 tissne, which are inter - con- 

 nected in such a way as to form 

 a kind of sponge-like structure. 

 New lamelljB are continually 

 being put forth by the formative 

 zone of the gland, which is a 

 narrowed portion bent back 

 downwards, while the oldest 

 lamellfe, which lie nearest the 

 aperture of the gland, become 

 detached and degenerate. All 

 the lamellse are covered by a 

 glandular epithelium and the 

 formation of the pigment can 

 be traced in all its stages from 

 its appearance in the epithelial 

 cells of the formative zone to 

 its condition in those of the 

 oldest lamellae. In the forma- 

 tive zone, the young glandular 

 cells are at first colourless. In 

 the succeeding lamellae, how- 

 ever, pigment granules increase 

 in number and from the older 

 lamellae are emptied into the 

 cavity of the gland, the epi 

 thelial cells then becoming 

 detached and breaking up. 



Both the gland and the reser- 

 voir are surrounded by a vascul 



The latter, after being formed 



Pio. Iij7.— Morphology of the pigment gland (ink-bag) of 

 the Cephalopoda (after P. Girod). A, Median longitudinal 

 section through the ink-bag of an adult, c. Anus ; 1, terminal 

 portion common to the rectum (2) and the duct of the inlc- 

 bag ; 3, ampulla ; 4 and 5, sphincter muscles of the ampulla ; 

 6, duct of the ink-bag ; 7, pigment reservoir ; S, opening of 



arised integument of connective ""* pigment gland into the reservoir ; 9, portion of the gland 



tissue ; 



,1 . , , traversed by lamelliie ; 10, formative zone of tlie lamellae. 



tne same integument ^^ various stages in the development of the pigment 



forms the framework of con- giand ; B, anal papilla ; 0, inv.igination in the same ; D, ap- 



nective tissue running through pearance of two new depressions at the base of C ; these 



increase in depth, the one becoming the pigment gland b, the 

 other the rectum 2. In F, the formative zone has appeared 

 in the gland, in G, the first lamellfe and the duct. H, I, K, 

 changes in the relative positions of the rectum and gland in 

 the course of development, seen from the posterior (mantle) 

 side. In H, tlie rectum lies behind the ink-bag. In I, the 

 latter has shifted, and in K lies behind the rectum (on the 

 mantle side). 



the lamellfe or trabeculse within 

 the gland. 



The ink-bag is further envel- 

 oped as a whole in a tough integu- 

 ment consisting of three layers : 

 (1) an inner glittering silvery 

 layer (argentea), similar to the 

 corresponding layer in the outer integument ; (2) a central muscle layer (inner 

 longitudinal and outer circular muscles ; and (3) an external layer of connective 

 tissue. 



The terminal ampulla has, at its two narrow ends, folds projecting inward and 

 functioning as valves ; it can be closed at these parts by sphincter muscles. The 



