32 MR. W. K. HUTTON ON THE [ Jan. 20, 
Avound the anus and the external nephridial apertures, both 
layers furnish special sphincter-fibres ; while along either side of 
the nerve-cord, as is afterwards noted, there runs a slender longi- 
tudinal strand of muscle-fibre (N.C., fig. 6). 
The general arrangement of the four special retractor muscles 
of the introvert and of the radial rectal muscular attachments 
have already been described ; there remains only, in regard to the 
former, to speak of the special appearance presented by the 
muscular fibres in this worm. On comparing the ventral 
retractors with the general longitudinal musculature, one is 
struck by the great similarity of texture presented by both. A 
glancing, bright suggestion of firmness, reminding one of the 
surface of fresh tendon among the higher forms, is noticeable. 
The dorsal retractors, on the other hand, have a dull, almost a 
translucent appearance. Microscopical examination of transverse 
sections reveals little difference between the two sets of muscles, 
save only that in a given area fewer muscular fibres are present 
in the dorsal muscles, and the amount of Intermuscular connective 
tissue appears to be greater than in similar sections of the ventral 
set (Plate VII. fig. 7, A & B). On the whole, both from the 
relatively marked shortness of the dorsal muscles and from the 
appearances just described, I incline to suspect that they cannot 
be functionally very active. They contain a sufficient amount of 
connective-tissue between the proper muscular bundles to make 
the suggestion that they are in a state of metamorphosis and 
regression at least probable (wide J, Bland Sutton’s ‘ Ligaments,’ 
&e., 2nd ed., 1897, p. 3 et seq.). 
Alimentary Canal. 
Surrounding the mouth, which hes at the bottom of the depres- 
sion formed by the union of their bases, are several simple 
leaf-shaped tentacles. So far as could be ascertained by the 
examination of a series of sections, 15 tentacles are present, and 
of these 11 are longer than the others. Hach tentacle contains a 
branch of the blood-sinus and a slender nerve (Plate VII. fig. 8). 
This figure, which represents a typical transverse section of a 
tentacle, shows that on the mesial aspect the cells covering its 
surface are narrow, long, arranged in palisade form, and are 
ciliated, while those of the opposite side form a simple columnar 
epithelium. For the greater part of their extent the tentacles 
are free, but round the mouth their bases are fused together, and 
form a series of gutters which lead to the oral aperture. 
The mouth itself lies on the top of a papilla, whose sides are 
grooved by the above-mentioned furrows. The cells lining the 
grooves are cubical and ciliated, and are continuous with the 
ciliated epithelium on the mesial aspect of the tentacles. Succeed- 
ing the mouth is a short cesophagus with very muscular walls; in 
section (Plate VII. fig. 10) it appears to be star-shaped, the lumen 
of the tube being encroached upon by the inward projection of a 
