18 A MONOGRAPH OF THE 



The latter are most developed in the Dorididce, but are never large, and are not always 

 present; the eyes being frequently supplied by simple nerves. The olfactory ganglia are 

 never absent, and are occasionally of considerable magnitude ; in some of the Eolididce they 

 are half the size of the cerebroids, and in Tritonia too they are very large. In the Dorididce 

 they are sessile; but they are removed to a considerable distance from the cerebroids, 

 by the interposition of long nerves or commissures, in most of the Tritoniadce and Eolididce. 

 In these cases, the olfactory ganglia are placed just within the base of the tentacles. 

 The auditory sacs are also in connexion with the cerebroids, upon which they are seated ; in 

 the Dorididce and Tritoniadce, on the under side near the commissure uniting them to the 

 pedials ; in the Eolididce, on the upper surface immediately behind the eye. In this respect it 

 would appear that the animals of this order disagree with the other gasteropods, in which 

 the auditory sacs are attached to the pedial ganglia. In the Heteropods, however, they are 

 always, as in the Nudibranchs, connected with the cerebroids. The cerebroids, as already 

 stated, give off the cords or commissures, which unite the oesophageal ganglia to the buccal. 



The branchial ganglia distribute nerves to the mantle or dorsal skin, and to the 

 branchial ganglia of the sympathetic system ; and, in the Eolididce, give off from the under 

 side, a nerve or two in connexion with the sub-oesophageal commissure or branchial collar, 

 which go to the viscera. In the Dorididce there are three or four visceral nerves which 

 originate in the visceral ganglion, attached to the right branchial. All these nerves go to the 

 chief centres of the sympathetic system of the viscera. In some of the Polycerince, however, 

 the genital nerve comes off from the side of the right branchial ganglion. 



The pedial ganglia send all their nerves to the foot.* 



The buccal set of ganglia are generally two pairs : — the buccal proper, and the gastro- 

 esophageal. The buccal pair, as already stated, are connected by commissure with the 

 cerebroids. The gastro-cesophageal are united to the buccal, upon which they are generally 

 sessile. The latter give their nerves to the buccal mass and tongue, and are connected with 

 the sympathetic plexus on the buccal organ. The gastro-cesophageal supply the salivary 

 glands, and send two large nerves — the par vagum, — down the oesophagus, which terminate in 

 two of the principal ganglia of the gastro-hepatic plexus of the sympathetic system. 



Sympathetic System. This is composed of numerous minute, variously formed ganglia, 

 connected together by open plexuses of nerves, spread out over the viscera. The principal of 

 these plexuses are, — the oesophageal, the gastro-hepatic, the pyloric, the intestinal, the 

 branchial, and the genital. Similar ganglia and nerves have also been found in connexion 

 with the heart, arteries, buccal mass, and skin. This system, as we have seen, is connected 

 with both sets of cephalic ganglia, with the oesophageal, through the visceral and branchial 

 centres, and with the buccal by thenar vagum, and gastro-cesophageal ganglia. 



This portion of the nervous system has been fully examined only in the Dorides, but 

 extensive traces of it have been found in the Poly carina, Tritoniadce, and Eolididce j and in the 

 latter family a very striking modification of the buccal plexus exists. In Eolis and Fiona, a 

 large nerve is given off on either side from a small ganglion, buried in the muscles of the 

 buccal mass, and is also in connexion with the buccal ganglia. These nerves supply the 



* In our description of the anatomy of Eolis, one of the pedial nerves is erroneously stated to 

 supply the skin of the sides of the back. 



