DORIS, 



these supra-oesophageal centres, is that which forms the third or great oesophageal collar (i). 

 It is stout, being composed of three parallel cords, and closely invests the oesophagus. 



The infra-cesophageal ganglia are two pairs, symmetrically disposed on the buccal mass : 

 the large or buccal ones {e, e) are, as above related, connected, by means of a collar, with the 

 cerebroid; they are elliptical, and united across the median line by a stout commissure. 

 They give off laterally, and in union with the collar, two pairs of nerves, the sixteenth and 

 seventeenth, which are distributed to the buccal mass. The eighteenth is a small pair which 

 passes off backwards from the same ganglia to the base of the tongue, and is joined by the 

 buccal filament of the sympathetic. To the front of the buccal are attached the gastro- 

 esophageal ganglia (/,/), which are very small, and give off three pairs of nerves. The 

 smallest of these, the nineteenth, is given to the salivary glands ; the twentieth supplies the 

 top of the oesophagus, and communicates with the oesophageal sympathetic plexus. Lastly, 

 the two nerves constituting the twenty-first pair, by far the largest of the three, are continued 

 down the under surface of the oesophagus, nearly parallel with each other, communicating by 

 slender filaments with a fine open network of nerves and ganglia upon that tube, and unite 

 with two of the largest ganglia of the sympathetic system of the stomach. 



The great supra-oesophageal ganglia vary a little in some species from those of 

 D. tuberculata, from which the above description is mainly taken. In D. pilosa and D. repanda 

 they are as distinct from each other as in that species, being only slightly altered in relative 

 position. In D. Johnstoni, D. coccinea, D. bilamellata, and D. aspera, the cerebroid and 

 branchial are completely fused into one mass, which in some of these species is elongated in 

 the antero-posterior direction ; in others, obliquely. No material variation takes place in the 

 origin and distribution of the nerves. The visceral ganglion is present in all. 



The sympathetic or organic nervous system is extensively developed in D. tuberculata: 

 it is more or less demonstrable in the skin, the buccal mass, and in all the internal organs. It 

 consists of a vast number of minute but distinct ganglia, varying in size and form, the larger 

 quite visible to the naked eye, of a bright orange colour, like the ganglia round the oesophagus, 

 and interconnected by numerous delicate white nervous filaments, arranged in more or less 

 open plexuses or networks. This beautiful system is, at several points, as already indicated, 

 connected with both sets of cephalic ganglia, and is most completely displayed on the digestive 

 organs, forming at the cardiac extremity of the stomach a distinct circular chain or belt of 

 ganglia and commissures. Ganglia and nerves, forming an extensive plexus, are also well 

 developed on the reproductive organs, and the branchio-cardiac (PL 1, fig. 2q) portion of the 

 sympathetic is equally conspicuous. 



Extensive traces of the sympathetic system have been detected in several other species, 

 as well as in D. tuberculata ; but on account of their diminutive size, the ganglia and 

 connecting nervous plexuses could not be followed with the same degree of accuracy as in 

 that species. 



The eyes of Boris are generally connected to the optic ganglia each by a very short 

 nerve, and are situated below the skin. In D. bilamellata, D. aspera, and D. pilosa, the nerve 

 is, however, somewhat elongated. The eye (PI. 2, fig. 14) evinces considerable development, 

 being furnished with a well-rounded black pigment-cup, having within the orifice a spherical 

 crystalline lens, which has in front an arched cornea, and the whole is enveloped by a trans- 

 parent membranous capsule. The auditory capsules (fig. 15) are always supplied with 



