71 



The entire wall of the pharynx and oesophagus, so far as it is exposed to the bod)-- 

 cavities, whether of the collar or of the trunk, possesses a single layer of fibres. I have never 

 observed the slightest indication of any crossing of fibres, although the muscular system can be 

 seen with great clearness in sections stained with iron-haematoxylin, particularly where parts of 

 the wall of the alimentary canal are cut tangentially, as in fig. 151. It need hardly be pointed 

 out that where sacculations of the wall occur, the direction of the fibres may appear to change ; 

 but their real course seems to me to be exclusively longitudinal. The pleurochords of Masterman 

 are in no way different from other parts of the pharyngeal epithelium, so far as the muscular 

 layer is concerned. But when the region of a gill-slit is reached, the fibres take a sphincter-like 

 course round the posterior or external wall of the tube leading to the external gill-pore. These 

 sphincter-like fibres are a direct continuation of the longitudinal fibres on the part of the pharynx 

 which is nearer the stomach, and of the longitudinal muscles of the body-wall of the trunk. 

 On the anterior or median border of the gill-slit, the fibres contained in the anterior horn of 

 the third body-cavity (fig. 156, d.cJ'a.) no doubt assist in the closure of the gill-pore. 



I have not noticed any differences of importance between the muscular systems of 

 different species of Cephalodiscus, except so far as has already been pointed out in describing 

 the stalk. The neuter individuals of C. sibogae are, however, provided with muscles which seem 

 to be specially strong in proportion to the size of the animal. 



XIII. NERVOUS SYSTEM. 



In 1 88 7 I was able to shew that the nervous system of Cephalodisctis conformed to 

 the type found in Balanoglossus. Since then, more detailed accounts have been given by 

 Masterman (97, 2 ; 98, 2 ; 03), many of whose results I am able to confirm. 



Masterman (97, 2, p. 342) recognises the following parts of the nervous system of 

 Cephalodiscus : (I) the central nervous system; (II) its backward continuation as the lateral 

 nerves, which are prolonged down the body as (III) a pair of lateral nerves similar to those 

 of Phoronis\ (IV) a post-oral ring given off from (II) at the posterior edge of the collar, and 

 becoming lost ventrally in the operculum, over which it forms a nerve-layer, (V) a broad tract 

 lying beneath the thickened anterior ectoderm of the proboscis, and connected with the central 

 nervous system round the "apex" or dorsal margin of that structure, (VI) a "pre-oral nerve" 

 passing ventrally along the posterior wall of the proboscis and originating from the anterior 

 part of the central nervous system by a root on each side; (VII) "several nerve-fibres" which 

 run down the anterior surface of the metasome, some of them being continued into the nerves 

 of the stalk; (VIII) the nerves to the arms. 



In a later account (98, 2, p. 513) Masterman substitutes for (VII) "a broad nervous 

 tract", which passes down the ventral line of the body into the stalk, while he states that the 

 lateral nerves (III) also extend into the stalk. 



There can be no question of the nervous nature of the parts numbered (I), (II), (VII), 



