BRITISH NUDIBRANCHIATE MOLLUSCA. 17 



the branchial, and the pedial ; the accessory, — the olfactory and the optic. They are placed 

 symmetrically with regard to the median line, there being one of each pair on either side. 

 The cerebroids are invariably above the oesophagus ; they hold a central position, and 

 are united by a very short commissure across the median line, being usually in contact with 

 each other, and, when distinct, are connected by commissures with the branchial and pedial : 

 these two latter of the same side also intercommunicate. The branchials, as well as the 

 cerebroids, are generally above the alimentary tube, and are frequently fused with them, so as 

 to form a single mass on each side of the median line. This is not uncommonly the case in 

 the Doridida, but seems to be universally so in the Tritoniada and Bolididce. The pedial 

 ganglia are also occasionally placed above the oesophagus ; but more frequently they are at 

 the sides, and sometimes, though rarely, they are below it, as in Doris pilosa, Ancula 

 cristata, Lomanotus marmoratus, and Doto fragilis. In the Dorididce, the visceral ganglion is 

 attached to the under side of the right branchial ; it is always small. In the Tritoniadce and 

 Eolididce this ganglion is apparently absorbed into the branchial. The fusion of the 

 oesophageal ganglia is most complete in Tethys lejjorina. In this species all the three pairs are 

 situated above the alimentary tube, and are so perfectly united into one mass, that it would be 

 scarcely possible to ascertain the boundaries of its component parts, were it not for the manner 

 in which the nerves are distributed. 



The oesophageal ganglia are united to the buccal, by two long commissural cords, which 

 pass from the under side of the cerebroids and embrace the gullet, forming a wide collar — the 

 buccal — around that tube. A large sub-cesophageal commissure, passing between the 

 oesophageal ganglia of either side, completes a second or great posterior oesophageal collar. 

 This commissure varies in length, in accordance with the position of the ganglia : if they are 

 all above the alimentary tube, it is long ; if partly below, very short. It is composed of either 

 two or three cords. In the Dorididce and Tritoniadce there are generally three, two of which 

 belong to the pedial ganglia, one to the branchial and visceral. This great oesophageal collar 

 is, therefore, in these two families really both a pedial and branchial commissure. In the 

 Eolididtz, however, it generally consists of only two cords, and they both come from the pedial 

 ganglia ; the branchial commissure is nevertheless invariably present, but is quite distinct 

 from that of the pedial, so that here the great posterior collar is represented by two separate 

 collars. Still there is apparently another sub-cesophageal commissure. Doris tuberculata has 

 a fine cord, uniting the cerebroid ganglia below the buccal mass. Thus, it seems probable 

 that in the Nudibranchs all the three principal pairs of oesophageal ganglia are united by 

 separate cords, below the alimentary tube.* But at present we must be satisfied with 

 enunciating the general facts ; that in the Eolididce the oesophagus is encircled by three 

 collars, namely: — first, the great oesophageal — the pedial; second, the branchial; third, the 

 buccal ; and that in the Dorididce there are usually two ; first, the great oesophageal, 

 comprising the pedial and branchial commissures ; and second, the buccal. 



The cerebroid ganglia give nerves to the channel of the mouth, to the lips and oral 

 tentacles, also to the veil, when it is formed by the fusion of these latter organs : they have 

 likewise attached to them the two pairs of accessory ganglia, — the olfactory and the optic. 



* This additional or cerebroid collar does not appear to be peculiar to the Nudibranchs, for we 

 have also detected it in Pleurobranchidium Meckelii. 



3 



