KEPORT ON CEPHALODISCUS DODECALOPHUS. 9 



connected. From this insertion they radiate into the thick central mass of the shield, 

 and some appear to reach the pale ventral region of the hypoderm. Somewhat behind 

 the former region the fibres also arise from the basement-tissue lining the h}^odermic 

 investment of the jaedicle. This fan-like arrangement of the muscular fibres must confer 

 great mobility on the disk, so that its broad scale-like surface can be applied either as a 

 sucker or in an undulatory or partial manner, indeed enabling it to act as a useful 

 locomotive organ. In this connection also it is probable that the basement-tissue may 

 be highly elastic, especially in the absence of any signs of horizontal or transverse 

 muscular fibres, and in connection with the entrance or exit of fluid by the proboscis 

 pores. The glandular nature of the disk, again, shows that it is a structure with 

 secerning powers of great activity, and in close relation with the remarkable ccencecium. 

 Superiorly the pedicle of the buccal disk runs into the region at the base of the arms, 

 and in sections a fine layer of longitudinal and oblique muscular fibres occurs on the 

 inner surface of the dorsal wall of the pedicle, though such have not yet been seen any- 

 where on the inner surface of the shield. 



In the centre of the buccal shield is a large mesoblastic cavity through which 

 the radiate muscular fibres before mentioned pass, and which communicates with the 

 exterior by two well-marked pores situated dorsally on each side of the middle line at 

 the great central nervous system. In oblique sections {e.g. PI. VI. fig. 3, h])) these pores 

 lie close together in their progress inwards. What relation the ciliated " sense " organ 

 of Rhahdo'pleura, as described by Sars, and also figured by Lankester, may have to the 

 proboscis-pores of Cephalodiscus is a feature of moment for futiire consideration. In 

 Cephalodisctis these pores seem to be formed by invaginations of the hypoderm of the 

 region, but their function is as uncertain as the single pore perforating the nervous system 

 in the proboscis of Balanoglossus. The proboscis in the latter is much more muscular and 

 has an evident proboscis-gland. 



While therefore the buccal disk of Cephalodiscus is in all probability the main organ 

 of locomotion, just as in Rhahdopleura, which was seen by Professor G. 0. Sars drawing 

 itself up to the aperture of its tube, it diSers from the shield of the latter by its much 

 greater size. 



In the form just mentioned the organ somewhat resembles the truncated and 

 thickened opercular process of certain Annelids, while in Cephalodiscus it overlaps 

 the neighbouring parts to a great extent. The intimate structure of the shield in 

 Rhahdopleura has only been alluded to by Sars, and he does not appear to have clearly 

 made it out. He says — " On examining more closely this buccal shield we observe in 

 the middle of it an opaque part which seems to contain an interior glandular organ. 

 Continuing the investigation and slightly pressing the animal, we notice, however, that 

 this opaque appearance is not produced by any such internal organ, but by a peculiar 

 and seemingly muscular structure of the shield itself It exhibits, seen from below, in 



(ZOOL. CHALL. EXP. — PART LXII. — 1887.) Qqq 2 



