196 COMPARATIVE ANATOMY CHAP. 



In the majority of Lamellibranchs, and in nearly all Diotocardia, the 

 rectum traverses the ventricle ; this fact, with many others, supports 

 the relationship of these two groups. 



In certain Molluscs, viz. the Scaphopoda, a few Prosobranchia 

 (Muriddce, Pwymridce), and the Cephalopoda, the hind -gut has an 

 accessory (anal) gland, which is well known in the Cephalopoda as the 

 ink-bag 1 . 



The rectal gland in Dentalium is a branched acinose gland opening into the hind- 

 gut, according to one account through six separate ducts, and according to another 

 through one single duct. Eggs and spermatozoa have been met with in the lumen 

 of this gland, and it has been supposed that they have been accidentally drawn out 

 of the mantle cavity by the swallowing-like action of the hind-gut, which has been 

 observed in Dentalium. 



The anal gland found in some Rachiglossa (Monoceros, Purpura, Murex) is always 

 dark in colour (brown or violet), and is either tubular with many bulgings of its 

 wall, or acinose with an axial duct. It always enters the hind -gut near the 

 anus. 



A gland has been found near the rectum in the Pteropoda thecosomata (Clio, 

 Cavolinia) and the Bulloida, and has been described as an anal gland, but this 

 requires further investigation. 



The ink-bag 1 of the Cephalopoda (Fig. 167), which is wanting only 

 in Nautilus, is a much developed anal gland. It enters the hind-gut 

 near the anus. The ink or sepia pigment secreted by it consists of 

 extremely minute particles which are ejected with vehemence from 

 the bag and discharged through the funnel. The pigment quickly 

 mixes with the water, and envelops the animal in a pigment cloud, 

 thus screening it from its enemies. 



Form and position of the ink-bag (cf. Figs. 160, p. 189 ; 177, p. 213 ; 178, 

 p. 214). The typical position of the ink-bag is in front of the rectum, i.e. in the 

 loop formed by the intestine in ascending from the mouth and then descending to 

 the anus. In Spinda, Enoplotcvtliis, and Scpioteuthis, the ink-bag is very small ; 

 it progressively increases in size in series both of Decapoda and of Octopoda, its 

 division into a saccular portion and a duct opening into the hind-gut in front of the 

 anus becoming more and more distinct. In the Octopoda, it lies embedded in the 

 upper part of the liver within the muscular hepatic capsule (cf. p. 128). It is still 

 found in this position (between the liver and the rectum) in Sepiola. In other 

 Decapoda, however, the ink-bag is found shifting higher and higher in the visceral 

 dome, its duct at the same time increasing in length. Finally, in Sepia (and the 

 fossil Dibranchia), it is found at the top of the visceral dome, behind the gonad. Its 

 duct runs along the right side of the hind-gut, bending round somewhat before 

 reaching the anal section of the rectum so as to enter the latter anteriorly. Onto- 

 genetically, however, even in Sepia, the ink-bag arises as an anterior outgrowth of 

 the rectum. 



Structure of the ink-bag in Sepia (Fig. 167 A). The ink-bag in this instance 

 consists of three parts: (1) the pigment gland which secretes the "ink" ; (2) the 

 pigment reservoir and the duct, which forms (3) an ampulla with a glandular wall near 

 its aperture. The pigment gland is a sac at the base of the ink-bag on its anterior 

 wall (that turned towards the gonad). It projects into the cavity of the ink-bag, 



