5 PHYSIOLOGICAL SERIES. 



these organs they do not seem to differ in kind from the post- 

 oral ganglia, and, except for a certain inhibitory power, cannot 

 be regarded in any special way as controlling or co-ordinating 

 centre* for the rest of the nervous system. Each pair of ventral 

 chain gnu States a reflex centre for its innervation 



awa. In the ventral chain of many Annelids there an> a 

 H m ii,,l number (usually three) of medullated giant nerve- 

 tube* : in manv instances connections have been seen between 

 them and giant or medium-sized cells upon the ventral 

 surface of the ganglia. Their function is still doubtful, but 

 they are probably nerve-elements and not simply supporting 

 structures. A definite visceral system is present connected 

 with the cerebral or subossophageal ganglia. In certain 

 form* the central nervous system is still closely united to the 

 epidermis (subcuticula), and in these cases the fibres of its 

 supporting tissues can be traced directly to the elongated ba>e.- 

 of the epidermal cells. 



CafiTOPODA. 



Raooritia, Arch. Zool. Exp., ser. 3, t. iv. 1896, p. 133 

 (Brain). 



D. 4. The anterior part of the body-walls of a Lug-worm 

 (Arfnifola marina) opened from the dorsal aspect. 



The nervous system, as might be expected from the 

 slnggi-h habits of the worm, is poorly developed. The 

 cerebral ganglion i- a -mall lobulated body situated, as in 

 other 1'olychfctcs, in the prostomiuin (in the specimen the 

 anterior part of the body-wall is turned inside out, so that 

 the cerebral ganglion and inverted prostomiuin form a 

 small excrescence above the cut edge of the pharynx). 

 There nr three distinct paired centres in the ganglion, 

 constituting a ton--, mid-, and hind-brain; each is situated 

 Iwneath and in clow contact with a particular sensory 

 area of the prostomial epithelium, upon which labial palp>, 

 tentacles and eyes, and a nuchal organ may respectively 

 be developed in higher Polycbsetes. The cerebral ganglion 

 is connected by a pair of long connectives (from which the 

 nerves to the otocyst* arise) to a ventral cord, that lies 

 within the body-cavity internal to the circular muscles; 



