74 



connexion with the collar and to have passed entirely into the metasome. It is thus hardly 

 correct to speak of them as passing in the posterior part of the collar along the whole of their 

 course to the ventral surface. Masterman considers that these nerves extend along the body 

 laterally, and he definitely compares them with the lateral nerves oi Phoronis. This is presumably 

 illustrated by fig. 17 of his paper published in 1897 (2). My own observations indicate that the 

 lateral nerves pass ventrally into the strong nerve-plexus which is seen on the anterior side of 

 the body in sagittal sections (PI. IV, fig. 42 and PI. XIII, fig. 181). Masterman's figure, to 

 which I have just alluded, shews the nerves correctly, but sections taken nearer the stalk would 

 probably have indicated that the paired nerves pass into an unpaired anterior nerve-tract. In 

 favourable specimens this course can be distinctly made out, although it is not always easy to 

 demonstrate it in specimens in which the epidermis is much stretched, and it is accordingly not 

 shewn in all my figures passing through the region in question. Figs. 119 — 124 (PI. X) shew 

 the lateral nerves, in frontal sections of C. kvinseni. They differ in no essential respect from 

 the corresponding structures in C. dodecalophus. Fig. 124, immediately ventral to the region 

 of the gill-slits, shews the ventral body-musculature along a considerable part of the anterior 

 surface. The lateral nerves have reached the posterior borders of the muscular areas. In the 

 next section figured (fig. 125) the nerve-layer is not so conspicuous as to be demonstrative, 

 and it is accordingly not represented in the drawing. But in similar sections of C. dodecalophus 

 it may be seen that the lateral nerves, after acquiring the position shewn in fig. 124, spread 

 out rapidly over the whole of the area of the antero-ventral musculature and become continuous 

 with the strong stalk-nerves shewn in fig. 133. The sagittal sections (fig. 181) demonstrate the 

 existence of a large nerve-plexus along the anterior side of the body from a region not far 

 behind the gill-slits to the stalk. 



The function of the lateral nerves is thus to place the musculature of the metasome in 

 direct communication with the central nervous system, a conclusion which is in accordance with 

 the great development of these nerve-tracts. 



C. gracilis and C. sibogae are less favourable for the examination of the nervous system 

 than the other two species, in consequence of their less robust structure. The main features of 

 the nervous system, as above described, can, however, be recognised in these species, and I 

 am not able to point out any differences of importance. 



It is a point of special interest that the reduced male individuals of C. sibogae (figs. 79, 

 81 — 84) have a well developed central nervous system, indicating that they must be regarded as 

 more than mere reproductive appendages of the colony. The development of the nervous system 

 is perhaps principally correlated with the functions of the proboscis and of the pair of arms. 



XIV. VASCULAR SYSTEM. 



The credit of discovering this system in Ceplialodisciis belongs to Masterman, who in 

 his first account (97, 2, p. 350), described the following parts : — (I) a large cavity beneath 



