366 COMPARATIVE ANATOMY. 



between these extreme forms. The fact that these enteric caeca 

 always have a glandular investment is of importance as bearing 

 on their real character. These branches, therefore, are not merely 

 physiological equivalents of a liver, but are to be regarded as modi- 

 fications of a true liver, which have come to take a share in increasing 

 the enteric canal, owing to the enlargement of the lumen of their 

 canals. The same organ, which in other Gastropoda has the appear- 

 ance of a liver, becomes a part of the digestive tract in the yEolidia?, 

 and retains its primitive significance in its walls only, or, it may be, 

 in parts only of these walls. The share taken by the cavities derived 

 from the enteron in the functions of this tract explains how it is 

 that the mesial enteric tube is so short. In other divisions also of 

 the Opisthobranchiata the liver has the form of wide tubes — as in 

 Phyllirhoe, Limapontia, etc. In all these structures the liver appears 

 to be degenerated, and there is no early stage of differentiation ; this 

 is because the iEolidia3 are derived from shelled Gastropoda. 



In the Pteropoda the liver is broken up into a large number of 

 small caecal tubes. In Pneumodermon these tubes arc collected 

 together into branched groups, and the wide orifices of their ducts 

 are scattered on the wall of the stomach, and give it an almost 

 sieve-like appearance. In the rest of the Pteropoda acini of simpler 

 character beset a portion of the enteron ; they form a well-defined 

 mass, which is traversed by this tube (Pig. 201, h). 



The liver of the Cephalopoda is always a large, and, frequently, a 

 compact gland ; in Nautilus it consists of four loosely connected 

 lobes, each of which gives off an efferent duct. In the Dibranchiata, 

 there are only two lobes, and these are either distinctly separated 

 (Sepia), or only partly connected together (Rossia). In Sepiola 

 and Argonauta the two lobes are more intimately connected ; in the 

 Loliginidae and Octopoda they form a single mass, which is traversed 

 by the oesophagus. In no case does the liver give off more than 

 two efferent ducts, which point to the primitive existence of two 

 lobes, and which, just as in Nautilus, always open into the end of tho 

 ccecum. 



There are peculiar glandular lobules on the hepatic ducts at the 

 openings into the caecum, and also in the liver itself ; these differ in 

 structure from the acini of the liver. The glands at either of these 

 spots have been regarded as pancreatic glands, but it must be 

 noted that they have no close affinity to the organs of that name 

 found in the Vertebrata. In the Gastropoda also (Aplysia, Doris) 

 special glands have been observed in the neighbourhood of the liver. 



3) Appendages of the Hind-gut. 



§ 282. 



Various glandular organs belonging to this division are found in 

 the Gastropoda only, where their significance has not been made 



