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 ganglia form an extremely flattened cir- 
cular dise, 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 ganglia being com- 
pletely fused in the median line. 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 ganglia, 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 
ganglia 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 (1898), 
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 Vayssiére 
(1885) for Notarchus punctatus. 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 Vayssiére, is the cerebro- 
buecal connective, which is not listed 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 ganglion, while 
the fifth cerebral nerve arises as two entirely separate trunks, c. a and ec. 5b. 
Other than in these points the cerebral nerves are similar to those of the alli : 
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, 
