MUSEUM OF COMPARATIVE ZOOLOGY. 165 



large nervous supply it receives from the brain direct. The cilia un- 

 doubtedly aid in propulsion of food particles into the mouth. 



3. Nervous System. 



a. Brain. 



The supraoesophageal ganglion {gn. su^oe., Fig. 3) lies dorsal to the 

 blood sinus, between the two dorsal retractors, and is enveloped by an 

 investment of connective tissue. The posterior surface (Plate II. Fig. 

 22) is marked by a considerable incision in the median plane, and the 

 anterior dorsal margin bears numerous digitate processes, which project 

 into the coelomic liquid. In sagittal sections the brain appears nearly 

 flat on its ventral side, whereas the dorsal aspect is considerably curved. 

 As seen from transverse sections, however, the dorsal surface is plane, 

 while a deep median furrow (Plate III. Fig. 25) penetrates the ventral 

 wall. Posteriorly (Fig. 25) this is continuous with a partition which 

 divides the brain into two symmetrical lobes. Anteriorly (Fig. 24) the 

 partition fails, and the division is only indicated by the furrow. Ou tlie 

 antero-veutral surface is the termination of the cerebral canal {cf. infra). 



The entire ganglion is covered by a capsule (Fig. 25, cj^s. enc), the 

 origin of which can only be determined by the consideration of a series 

 of transverse sections. Following such a series from a short distance 

 posterior to the brain, it will be seen that the septum joining the two 

 dorsal retractors is here fused with the dorsal wall of the oesophagus, in 

 which lies the dorsal vessel. As the posterior extremity of the brain is 

 reached, this septum rises upon the brain, covering its dorsal aspect, and 

 still showing laterally the connection with the dorsal retractors. Imme- 

 diately inferior to the brain lies the dorsal vessel (Fig. 25, vn. sng. d.), or, 

 anterior to this, the blood sinus, which is separated from the brain only 

 by its own wall, which thus forms the ventral covering of the brain. 

 The dorsal and ventral layers of the capsule are continuous on those 

 parts of the lateral aspect of the brain where there are no outgoing 

 nerve stems ; but when the latter exist, a neighboring portion of the brain 

 capsule is reflected over them to form the neurilemma (Fig. 25, coii'f. tis.). 

 This composite capsule is made up of a loosely woven mass of fibres which 

 often show a plaited arrangement. The discoid nuclei measure 3.5 by 5 fx, 

 and are deeply stained in all coloring fluids. Inferior to this basketwork 

 of fibres are found occasional nuclei, which stain very faintly and pos- 

 sess each a few small nucleoli. These are surrounded by a small amount 

 of a granular substance, and are very similar to nuclei found in the midst 



