43 



A mechanism of tliis sorl would recjuire some power of closiiiu- either one or both of the 

 apertLires of each canal, as occasion requires. The condition in which the canals are left in 

 preserved specimens is, however, usually one in which both external and internal apertures are 

 widely open. 



The consideration of these (piestions must be preceded by a more careful study of the 

 collar-canals themselves. 



The external aperture of a collar-canal {cc. e.) is seen in PI. III, fig. 25, which is drawn 

 from a plasticine reconstruction of the sections belonging to the sagittal series shewn in PI. V. 

 The external part of the opercular flap is cut away on the right side, and immediately below 

 this part the collar-pore appears as a projection of the body-wall in the form of half of a 

 shallow cup. Below the collar-pore is seen the aperture of the corresponding gill-slit {g. s. r.). 

 Figs. 50 — 46 shew consecutive sagittal sections of this collar-canal. It appears from these sections 

 that the external aperture of the canal is everywhere overhung by the base of the operculum, 

 and the arrangement suggests that this structure when reflected towards the stalk of the animal, 

 might be used for closing the collar-pore. There is also clear evidence of a tissue passing from 

 the thin dorsal wall of the collar-canal to the base of the epidermis of the oral side of the 

 operculum. If this tissue were muscular the collar-canal would appear to be provided with a 

 dilating mechanism. 



But in most other cases I have not been able to demonstrate any connexion between 

 these supposed dilator fibres, which I will term the "problematical tissue" of the collar-canals, 

 and the oral epidermis of the operculum. 



Two sets of muscles are, however, intimately associatecl with the collar-canals. The first 

 of these are the great antero-ventral muscles of the body, continuous ventrally with those oi 

 the stalk. These muscles (PI. X, figs. 125, iiis.) divide into two masses when they reach the 

 level of the ventral border of the collar (fig. 124), the fibres becoming fewer in number ; and 

 they diverge outwards as they cross the gill-slits on their anterior side (fig. 123), finally 

 disappearing when they reach the trunco-collar ^) septum in the neighbourhood of the inner 

 openings of the collar-canals. These antero-ventral horns of the third body-cavities {ó. c'. a.) are 

 seen in longitudinal section in PI. IV, fig. 41 (C. dodecalophus) and in PI. IV, fig. 36 (C levinseni). 

 They are essentially alike in all the species which I have examined. 



The second set of muscles are the strong oral muscles [or. >n.) which traverse the collar 

 in a longitudinal direction. These muscles (PI. IV, fig. 41 ; PI. V, fig. 45; PI. VIII, fig. 93) 

 originate from the trunco-collar septum close to the insertion of the muscles contained in the 

 anterior horns of the third body-cavity, and pass dorsally round the sides of the mouth, ending 

 in septum '/, opposite the point of origin of the radiating muscles which pass freely through the 

 cavity of the proboscis. 



Since the ventral wall of the collar-canal is firmly attached to the septum with which 

 these two sets of muscles are connected, it would appear that contraction of the muscles must 

 have some effect on the collar-canal. This organ has a perfectly characteristic relation to the 



i) I bonow this expression from de Selys Longch.\mfs (04). 



