ALIMENT AEY CANAL OF CEPHALODISCTJS. 43 



the post-oral lamella, the snapped threads drawing themselves and their nuclei up to 

 the coelomic surface of the muscular fibres. It is a significant circumstance that while 

 a greater or smaller number of perfect coelomic trabeculae are found in most parts of 

 the collar cavity and the cavities of such out-growths from it as the plumes and post- 

 oral lamella, only broken fibres are found projecting from that part of the post-oral 

 lamella which lies immediately ventral and anterior to the collar canal muscle 

 (text-fig. 13). 



The present species possesses the advantage of superior size over those species 

 examined by Harmer, and is consequently well adapted for the determination of the 

 nature of the " problematical body." The muscle-fibres composing it can even be 

 recognized as of the "cross-striped" variety (see text-fig. 16). It is worthy of 

 remark that the collar canal muscle is the only cross-striped muscle in the body of 

 Cephalodiscus. While cross-striped muscle fibre is widely spread throughout the 

 Vertebrates and occurs in considerable bulk, in the Invertebrates it occurs only in 



TEXT-FIGUBE 16. Section of the collar canal muscle of C. nigrescens cut parallel to the ventral surface of the polypide, 

 i.e., parallel to the face of the buccal shield ; as seen with Zeiss apochr. imm. obj. 2mm., compens. oc. 6 ; x 820. 



isolated genera or families, and only in certain limited tracts of the body. A higher 

 muscular efficiency and a more rapid contraction appear to be associated with this 

 histologically differentiated muscular tissue, and one is led to the conclusion that 

 the collar canal of Cephalodiscus is an organ of considerable importance. 



Alimentary Canal. 



The alimentary canal differs from that of C. dodecalophus, mainly in that the 

 stomach is not globular. The canal is in most cases empty, or nearly so. The 

 first part of the tube (pharynx) has a thick wall, and is lined with ciliated 

 epithelium. Projecting forward from its anterior wall is the notochord, a long, 

 narrow, blind tube, with continuous lumen opening into the pharyngeal cavity, and 

 with walls composed of pale-staining, vacuolated cells. The two gill-slits, right 

 and left, are small oval apertures, symmetrically disposed and situated not far 



