330 THE DOLABELLINAE. 



the posterior end of the visceral loop, while other smaller ganglia, such as those 

 of the anterior tentacles and rhinophores are borne at varying distances from 

 the central group. 



Cerebral ganglia. — The cerebral gangha form an extremely flattened cir- 

 cular disc, lying upon the dorsal face of the anterior end of the oesophagus, just 

 as it emerges from the pharyngeal bulb. Since they are not closely united 

 to the underlying digestive tube a certain amount of variation in their position 

 in reference to it may be found in different specimens, as pointed out by Lacaze- 

 Duthiers (1898). They are inclosed in a tough sheet of connective-tissue, which 

 forms a close envelope about them, and is prolonged out upon the nerves, and 

 in a web-like expansion connecting them. Thus the actual size of the ganglia 

 is obscured, but they approximate 30.0 mm. in length and breadth, the thickness 

 being 1.5 mm. in their somewhat shrunken condition. Including the sheath 

 the antero-posterior length is 60.0 mm., the lateral diameter is 70.0 mm. 

 Nothing can be seen of the cerebral commissure, the paired gangha being com- 

 pletely fused in the median hne. Plate 2 represents a dorsal view of the cen- 

 tral circumoesophageal group of the nervous system of D. agassizi, with the 

 exception of the buccal gangha, which are shown, Plate 3, fig. 3. The con- 

 nective-tissue sheaths have been dissected away, and have been omitted entirely 

 from the drawing for the sake of clearness. All of the nerves issuing from the 

 gangha appear much wider before this sheath is removed, than their actual 

 dimensions warrant. 



Cerebral nerves. — In designating the cerebral nerves the endeavor has been 

 made to use the same serial numbering as that employed by Mazzarelli (1893), 

 and by the writer (1909) for Tethys dactylomela and T. cervina, and to homologize 

 it with that adopted by Amaudrut (1886) for Dolabella, and by Vayssiere 

 (1885) for Notarchus pundatus. The following tabulation (p. 33) of the cerebral 

 nerves and their distribution in these forms will render a comparison simple. 



The first cerebral nerve in Notarchus according to Vayssiere, is the cerebro- 

 buccal connective, which is not hsted in the above table. In Dolabella agassizi, 

 as will be seen by reference to Plate 2, fig. 1, the first and third cerebral nerves 

 are united into a common trunk, A, for some distance from the gangUon, while 

 the fifth cerebral nerve arises as two entirely separate trunks, c. 5a and c. 5b. 

 Other than in these points the cerebral nerves are similar to those of the allied 

 forms given in the table. 



The nerve A of Plate 2, fig. 1 is the largest of the cerebral nerves of Dola- 

 bella. It arises from the anterior border of the ganglionic mass on either side. 



