192 



COMPARATIVE ANATOMY 



CHAP. 



I 



each side of narrow septa arranged at right angles to the gut, or transversely (Fig. 163) ; 

 in these septa, muscle fibres run down to the rudimentary foot, and blood lacunre 

 abound. In Proncomenia Sluitcri, septa of the first, second, third, or fourth order can 

 be distinguished, as seen in the figure. The septa on the right alternate with those 

 on the left side of the body. In the dorsal middle line the mid-gut forms a narrow 

 ciliated longitudinal groove which cuts deep into the gonad, cilia are also found on its 

 medio- ventral surface. 



2. Gastropoda. 



The digestive gland of the Gastropoda falls into two or more lobes, between 

 which the stomach and the coils of the small intestine lie embedded. One, two, or 



more ducts of the gland may open into the 

 stomach. The walls of the digestive gland 

 show the same division into layers as the wall 

 of the alimentary canal. For details as to the 

 ferment, hepatic, and calcareous cells forming 

 the epithelium of the gland, and their physio- 

 logical constitution, the reader must be re- 

 ferred to special histological and physiological 

 treatises. 



In the Nudibranchia, as already mentioned, 

 the digestive gland breaks up into a system of 

 glandular diverticula (the so-called "diffuse 

 liver "). The AeolidiadcK (e.g. Tcryipcs] afford 

 an instructive instance of this. Three diver- 

 ticula rise from the stomach, two anterior and 

 lateral, and one posterior and unpaired. These 

 ramify in the body cavity, and finally send up 

 their last ramifications or lobes into the dorsal 

 appendages. The contents of the intestine can 

 penetrate into these last ramifications of the 

 " diffuse liver " (Fig. 164). 



Further, within the Nudibranchia the break- 

 ing up of the compact digestive gland to form a 

 "diffuse liver," i.e. the loosening from one 

 another, and the spreading out of the glandular tubes which are in close contact in 

 the compact gland, can be followed almost step by step. In the Tritonidce the gland 

 is a great compact mass. In other families, such as the Tcthymelibidce, Lomanotidce, 

 Dendronotidcc, Bomellidcc, Scyllccidce, it divides into two anterior accessory livers 

 and a posterior principal liver, from which diverticula run up into the dorsal append- 

 ages. Finally, the accessory and principal livers break up into separate "hepatic 

 branches" (Aeolidce), which in some cases anastomose. The posterior principal 

 branch of the "diffuse liver" gives off specially numerous lateral branches ; it often 

 widens out to a pouch, and may then be compared to an extended gall bag, or a 

 posterior diverticulum of the stomach. In Phyllirhoe, a pelagic form, without 

 dorsal appendages, the "diffuse liver" is simplified, consisting of four un branched 

 blind tubes, the two anterior opening into the stomach separately, the two posterior 

 entering it together (Fig. 19, p. 12). 



The stomach of many Opisthobranchia consists of two divisions separated by a con- 

 striction. In some forms, such as the Bullidce among the Tcctibranchia, the Ptero- 

 poda thccosomata, and the Tcthymelibidce, Borncllidce, Scyllceidce, among the Nudi- 

 branchia, it is armed with hard chitinous plates, spines, teeth, etc., occurring in 

 varying number and arrangement on its inner wall (Figs. 161 and 162) 



Fn;. liH. Alimentary canal of Aeolis 

 (after Souleyet). 1, Pharynx ; _', stomach ; 

 3, branched digestive gland (liver); 4, 

 anus ; 5, rectum. 



