69 



zooid of C. sibogae, and it shews the greater part of the course of one of these muscles. The 

 metasomatic musculature (/;/^-.) is cut tangentially, and is seen to pass into the septum between 

 the second and third body-cavities ^). From the anterior side of this septum, in the immediate 

 neighbourhood of the gill-slit [g. s.) and of the collar-canal [c. c.) starts the oral muscle (or. m.), 

 which passes through the collar-cavity as far as septum Vs, i'lto which some of its fibres appear 

 to be inserted. The muscle receives, however, an important contribution of fibres from the oral 

 side of the lower lip or operculum. 



Fig. 107 (PI. IX) is a part of a transverse section of C. levinseni which is favourably 

 orientated for the study of part of the origin of the oral muscle. The section passes through 

 the ventral side of the collar and metasome in such a way as to expose the surface of the 

 basement-membrane [p. in.) belonging to the oral epidermis of the operculum, which is directed 

 towards the proboscis. The section will be understood by referring to fig. 123, the bilobed 

 portion of basement-membrane marked d. m. being that of the anterior wall of the epidermic 

 recess at the base of the operculum. The muscle-fibres start from the ventral collar-mesentery 

 and pass in a transverse direction across the wall of this recess, and in the same direction 

 across the wall of the mouth (at the level ;;/.), below the end of the recess. It may be noted, 

 however, that the transverse fibres and the oblique fibres which are found on the anterior wall 

 of septum -/.; are parts of a continuous layer. In suitably stained frontal sections similar to 

 PI. XII, fig. 156, the constituents of the oral muscle may be seen, cut transversely, in the 

 angle of the collar-cavity on each side of the mouth, some in contact with the wall of the 

 anterior horn of the thii'd body-cavity and some in contact with the epithelium lining the mouth 

 or with that of the oral surface of the operculum. Here they are continuous with the layer 

 (not shewn in figs. 153 — 156) which occurs along the whole of the base of the epidermis of 

 both surfaces of the operculum, and they are in fact merely a specialised part of this layer. 

 Traced towards the dorsal surface (Figs. 154, 153) the oral muscle becomes more definite. In 

 fig. 151, it is seen in sagittal section receiving fibres from the oral wall of the operculum, 

 while in adjacent sections of the same specimen it receives other contributions from the basement- 

 membrane covering the anterior wall of the mouth or of the commencement of the pharyn.x "). 



The constitution of the oral muscle is thus complicated. It may be regarded as the 

 principal muscular pillar of the proboscis, but its base is spread over a wide area, and some 

 of its fibres probably act as an oral sphincter. The transverse cour.se of the fibres on the ventral 

 part of the operculum is doubtless of considerable functional importance if, as seems not 

 improbable, particles of- food pass into the opercular recess below the mouth before they are 

 swallowed. The transverse course of the muscles here situated probably enables the opercular 

 recess to empty itself by a sort of gulping action into the mouth. 



It is difficult to arrive at complete certainty with regard to the course of the muscle- 

 fibres in the operculum. In some parts, the fibres may be cut transversely in a sagittal section, 

 while in others their course is more oblique. In the projecting lateral lobe of the operculum 



1) It would haidly be possible from this section to prove that the septum dividcil the muscles of the two cavities; but the 

 evidence of other specimens seems to me clear on this point. 



2) It is not easy to say where the mouth ends and the pharynx begins. 



