REPORT ON CEPHALODISCUS DODECALOPHUS. 23 



interest connected with the condition of the permanent pedicle in Cephalodiscus. This 

 is a truly homologous process of the posterior body-cavities, and in sections at the base, 

 as already explained, the septum is visible {vide PI. V. fig. 4). It is further placed 

 ventrally in regard to the intestine, and its tip, so far as structure shows, is in all 

 probability also used as a sucker ; indeed, the buccal shield and the tip of the pedicle 

 form far more efficient and permanent sucking disks than ever occur at any stage in 

 Balanoglossus. It is the disappearance of the " tail " in the latter which alters the 

 character of its reproduction, and confines it so far as known to the sexual process. The 

 presence of this organ in Cephalodiscus, again, and its striking activity in bud-formation, 

 open up new fields in homology and assist in bridging over the gaps which formerly 

 existed between apparently isolated groups. . 



Nervous System. 



In the preliminary account of Ccphcdodiscus^ no distinct nervous system was 

 observed. In Mhabdopleura all that the careful observations of Sars could distinguish 

 in regard to a nervous system was that " immediately behind the anal aperture, between 

 the terminal part of the intestine and the dorsal wall of the gullet, which here forms a 

 little concavity, there appeared a clear cellular body (fig. 1 5, r) in which several evident 

 nuclei were visible. I cannot, however, pronounce any decided opinion as to the 

 significance of this object ; it can scarcely be a nervous ganglion, as it does not lie in the 

 substance of the body itself, but only in the thin external skin which encloses the body." 

 Lankester, again, considers that it is probable that the "clear cellular body" just 

 mentioned by Sars was a gonad. It is doubtful, however, whether the latter interpre- 

 tation can be maintained, especially when the condition now known in Cephalodiscus is 

 considered. At the base of the plumes in the latter, and situated over the median space 

 existing there — above the mouth — is in section a region (PL VI. fig. 3, nc ; PI. VII. 

 fig. 3, tic), which appears to me to contain the central nervous system of the animal. 

 The area is of considerable proportional size, and is minutely cellular and granular, while 

 fibrous bands stretching from it occur in other views. Its inner face rests on the thick 

 basement-layer bounding the collar-spaces at the base of the arms, and to the opposite 

 wall of which the muscular fibres of the great buccal disk are attached ; while its outer 

 covering is formed by a thick layer of hypoderm. . This region forms an elevation dorsally 

 between the bases of the arms (PI. VI. fig. 3, nc), and thus is in close communication 

 with these organs, while it is also within a short distance of the pigment of the oviducts, 

 though no branches have been traced to these organs. It extends a considerable distance 

 laterally on each side along the basal region, whence the plumes spring, and for some 

 distance on the dorsal surface of the buccal disk. The relations of the nervous system to 



' Ann. and Mag. Nat. Hist, ser. 5, vol. s. p. 337, 1882. 



