20 THE VOYAGE OF H.M.S. CHALLENGER. 



is further strengthened ventrally by a muscuhxr layer, but this will be described under 

 its special head The preparations showed traces of what might be an epithelial 

 layer on the inner surface of the before-mentioned basement-tissue, but such were far 

 from being distinct. In the living Rhabdopleura, ou the other hand, such an epithelial 

 layer is described by Professor Lankester, under the name of " enteric epithelium;" and its 

 distinctness in this form suggested its presence in Cej^halodiscus. 



Body-Cavity. — The foregoing layers enclose the body-cavity {co in sections of buds), 

 which is generally filled more or less completely by the alimentary canal. In the 

 preliminary account ' it was pointed out that this investment was probably homologous 

 with the " thin glassy skin " of Sars surrounding the digestive canal in Rhahdopleura, 

 and that the preparations gave no evidence of perigastric fluid. Though the existence of 

 a body-cavity was not specially noticed, the preparations did not warrant a denial of 

 its presence in Cephalodiscus, as Professor Lankester states in a recent paper," for thus 

 the hypoderm and basement-tissue must have been amalgamated with the coat of the 

 alimentary canal, which was not the case. This statement does not in any way detract 

 from the credit which Lankester has in clearly describing for the first time the chamber 

 in the living Rhahdopleura. Small nucleated corpuscles were occasionally seen in 

 groups in the cavity of Cephalodiscus in the sections, but they may have been intro- 

 duced from other sources. Neither Sars nor Lankester observed such in the living 

 Rhahdopleura. In sections the continuation of the body-ca-vdties in front are seen a 

 little behind the paired cavities connected with the lophophoral apparatus, and are 

 likewise surrounded by basement-tissue. 



Muscular System and Pedicle. 



As previously mentioned, the short ventral surface of the body is continued 

 into the cylindrical pedicle, which is invested by the hypoderm and basement-tissue, 

 the former being thrown into numerous and rather regular transverse wrinkles 

 in contraction, and being thicker dorsally than ventrally. At the terminal region 

 of the foot (PI. VI. fig. 1, hps) the hypoderm is much increased in thickness, 

 but has the same structure. It is free from the wrinkles which characterise 

 other parts of the region ; and appears indeed in favourable preparations to form a 

 flattened sucker-like disk. The basement-layer within the terminal hypoderm is thick, 

 and has attached to it the longitudinal muscular bands, so that it is possible it may be 

 occasionally used as a sucker like that of Loxosoma, or like the larval organ in 

 Balanoglossus. The entire pedicle within the basement-tissue is filled with the longi- 

 tudinal fibres, which arise on the ventral wall of the body in the region of the mouth, 

 where they present the form of a thinner lateral region and a denser central, the latter in 

 ' Op cit, p. 344. 2 Polyzoa, Ency. Brit., v6l. xix. p. 436. 



