VII 



MOLLUSCA^THE ALIMENTARY CANAL 



197 



wlucli serves as reservoir and duct for the pigment, 

 in the gland, passes through an 

 aperture in its wall into this 

 reservoir. Tlie cavity of the 

 gland is traversed by numerous 

 perfoi'ated and richly vascular- 

 ised lamellfe of connective 

 tissue, which are inter - con- 

 nected in such a way as to form 

 a kind of sponge-like structure. 

 New lamellfe are continually 

 being put forth by the formative 

 zone of the gland, Avhich is a 

 narrowed portion bent back 

 downwards, while the oldest 

 lamellse, which lie nearest the 

 aperture of the gland, become 

 detached and degenerate. All 

 the lamellfe 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 lamellfe. In the forma- 

 tive zone, the young glandular 

 cells are at first colourless. In 

 the succeeding lamella?, 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 

 detached and breaking up. 



Both the gland and the reser 

 voir are surrounded by a vascul 



The latter, after being formed 



Fiii. lor.— Morphology of the pigment gland (ink-bag) of 

 becoming *^^ Cephalopoda (after P. Girod). A, Median longitudinal 

 *" section thiougli the ink-bag of an adult, k, Anus; 1, terminal 

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

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

 (3, duct of the ink-bag ; 7, pigment reservoir ; 8, opening of 

 arised integument of connective *'^® pigment gland into the reservoir ; 9, portion of the gland 

 tissue ■ the same intpcrn t traversed by lamelbe ; 10, formative zone of the lamella?. 



° B-G, Various stages in the development of the pigment 



forms the framework of con- 



gland ; B, anal pa pi 



C, invagination in tlie 



D, ay 



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



the lauiellpe or trabeculfe within increase in ilepth, the one becoming the pigment gland h, the 



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



rS', . , , • f i.1 1 '" ^'"^ gl'*'"^') in Cr, the first lamelhe and the duct. H, I, K, 



ilie mk-bag is further envel- changes in tlie relative positions of the rectum and gland in 



oped as a whole in a tough integu- the course of development, seen from the posterior (mantle) 



ment consisting of three layers : ^v\&- In H, the rectum lies behind the ink-bag. In I, the 



(1) an inner cflitterinff silvi^rv i*^*'^'' has shifted, and in K lies behind the rectum (on the 



1 , ^ °v • ., , mantle side), 

 layer (argentea), smiiiar 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 



