292 MENSCH. [Vol. XVI. 



buccal cavity and not a separate cavity. In a median section 

 like the one represented in PI. XIII, Fig. 14, this space is not 

 seen, and the posterior part of the head in this plane is made 

 up entirely of the muscular structures of the dorsal median 

 tentacle (t.m.), which reach as far ventralward as the junction 

 of the epidermis with the endoderm. Anterior to the muscu- 

 lature of this tentacle is the brain structure, which consists of 

 a central brain mass surrounded by a dense mass of ganglion 

 or brain cells {c.n.). The central brain mass or medullary sub- 

 stance consists of two distinct lobes, the anterior of which (cb.a.) 

 is the smaller, and gives off a pair of nerves to the palps and 

 sends fibers into the circumoesophageal nerve ring. The poste- 

 rior lobe {cb.p.) is the larger, and is partly divided into two 

 smaller lobes by a fissure that extends more than two-thirds 

 across the lobe. It supplies the tentacles and eyes with nerves 

 and gives off fibers which form the greater part of the circum- 

 oesophageal nerve ring. Examined under a high power, this 

 central brain tissue is found to consist of very minute fibers 

 interlacing one another and having no distinct direction except 

 in regions from which nerves are given off, where they assume 

 a more parallel course. The tissues surrounding this central 

 brain substance consist of a mass of cells imbedded in a 

 network of very fine fibers, not unlike the fibers of the central 

 mass, but less densely arranged. The brain cells are not scat- 

 tered uniformly around the medullary substance, but are so 

 crowded as to form centers — these centers occurring at the 

 origin of the nerves and at the fissures in the medullary sub- 

 stance. The outlines of these cells, particularly in the region 

 of the centers, are very indistinct, and even away from these 

 centers it is difficult, by means of the ordinary methods em- 

 ployed, to distinguish the exact shape of the cells. In the 

 less dense regions of the eyes, however, as has been observed 

 by Malaquin in other Syllidians, distinct unipolar cells may be 

 distinguished. 



The epidermis overlying this cortical substance, just as in 

 the case in that underlying the nerve cells of the ventral cord, 

 cannot be distinguished from the cortical substance, the cells 

 of both being so intimately associated that the outline of the 



