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



however, is contrasted with them very considerable differences are encountered, one of 

 the most decided being the straightness of the canal and its terminal anus. 



Tlie Xervous System is prae-oral in Cephalodiscus and it has a somewhat peculiar' 

 structure. It is situated between the hypoderm externally and the basement-tissue 

 internally. None has yet been described in Rhabdopleura, and thus comparison at 

 }i resent must remain in abeyance. In Phoronis the nervous elements, which lie along 

 tiie base of the branchial processes, are similar to those in Cephalodiscus, and there is 

 also a central area between mouth and anus. The nervous system is subhypodermic, 

 and is chiefly concentrated in the corresponding region to that in Cephalodiscus. So 

 far as known, therefore, all have something in common under this head. 



Unless the oviducts in Cephalodiscus are to be credited with remarkable functions, 

 sen: e organs, so far as can be noticed in the spirit preparations, are absent. Further 

 inquiry is necessary on this head, but I am inclined to consider with Dr. Marcus Gunn, 

 on whose special experience and caution I place reliance, that there is no refractive 

 mechanism. This locaHzed pigment perhaps indicates either phosphorescent organs or 

 local heat-producers. In Rhabdopleura a pair of ciliated pads or papillae occur at the 

 bases of the plumes, the minute structure of which, however, is in need of careful 

 revision. Phoronis, again, presents only the ciliated furrows, which are external to the 

 anus, and have a nervous expansion beneath the hypoderm. In Loxosoma papillae similar 

 to those just described occur on the dorsal aspect, and the subcesophageal ganglion is 

 w^ell developed, as shown in Harmer's beautiful researches on Loxosoma crassicauda. 



The structure of the Body-wall in Cephalodiscus considerably diverges from that in 

 Rhahdojjleura, the definite layers of hypoderm and basement-tissue in this form 

 contrasting with the cuticular epithelium and underlying connective-tissue cells in the 

 latter. Something like basement-tissue seems to be indicated in Lankester's pi. xl. 

 fig. 12 (though no mention of it is made), and he describes and figures the enteric 

 epithelium lining the body-cavity, the cells being connected with the wall of the stomach 

 by processes. The body-wall in Phoronis again deviates from that in either of the 

 foregoing, since, besides cuticle, hypoderm and basement-tissue, it shows a circular and a 

 longitudinal muscular coat, the latter being chiefly grouped in longitudinal bands which in 

 transverse section show a somewhat pennate arrangement. There is little in common, 

 therefore, beneath the basement-tissue, and the absence of the pedicle in Phoronis is a 

 marked feature of divergenc-fe. The structure of the body-wall of Cephalodiscus most 

 nearly approaches that of Balanoglossus, though there is a wide gap in this respect as 

 there also is between it and Phoronis. In Loxosoma a transparent cuticle and a 

 hypodermic layer " associated at certain points with muscular fibres more or less 

 pronounced," according to Vogt are present. 



Cephalodiscus and Rhabdopleura agree in certain respects in regard to the Body- 

 cavity, but the former has the pedicle as an appendix. Phoronis on the other hand has 



