67 



pass fonvards as far as the region of the collar-canals (PI. I\\ fig. 41 ; PI. VIII, fig. 93), and 

 the last of the body-muscles are inserted into the septa wliich divide the ventral horns from the 

 collar-cavitv. 1 think there can bc little doubt that none of the fibres traverse the septum ; and 

 indeed it maj- be stated that the longitudinal muscles in each of the segments of Cephalodiscus 

 are restricted to that segment, just as tlie longitudinal muscles of Amphioxus or of a Fish are 

 divided into myomeres by the myotomes or myocommata. 



From the anterior border of septum '/;i, in the immediate neighbourhood of the termination 

 of the metasomatic muscles, originate the principal longitudinal muscles of the collar. These 

 consist of a pair of strong bundies of fibres, the oral muscles (PI. VIII, fig. 93 ; PI. X, figs. 

 122 — 117, ör. ;//.), which start from the region of the collar-canals, traversing the collar-cavity 

 on either side of the mouth, and ending, to some extent at least, in septum Va» opposite the 

 origin of the radiating muscles which pass through the cavity of the proboscis. I point out 

 below that these muscles are joined by fibres which have a different origin. 



The arms themselves have a pair of longitudinal muscles, running along the basement- 

 membrane of their morphologically inner side ; that is to say, along the side of the arm-grooves ; 

 and a pair of weaker longitudinal muscles extending along their outer side (PI. XII, fig. 141). 



The principal muscles of the proboscis form two strong groups of fibres which diverge 

 from the thick membrane which forms septum V3, and are inserted largely into the basement- 

 membrane of the anterior wall of the proboscis (PI. XII, figs. 147 — 153, PI. XI, figs. 137 — 139). 

 These have already been described by M'Intosh (87) and Masterman (97, 2), both of whom 

 have suggested that they enable the proboscis to be used as a sucker. 



It will thus be seen that there is a functional continuity between the stalk-muscles and 

 the longitudinal muscles of the anterior end of the animal. The contraction of this svstem will 

 not only shorten the stalk (as is indicated by the numerous transverse wrinkles into which its 

 wall is commonly thrown), but will also shorten the body, the collar as a whole, and its arms 

 individually, and will retract the anterior body-wall of the proboscis. This general contraction 

 is presumably associated with an attachment of the sucker-like base of the stalk to the wall 

 of the coenoecium. The contractions are no doubt of importance, not only in the movements 

 of the entire animal, but also of its separate parts. The movement of the proboscis, for instance, 

 whether in crawling or in tube-building, must be largely controlled by the muscles which traverse 

 its body-cavity. 



The first two segments of Cephalodiscus probably have a mechanism similar to that 

 of the tube-feet of an Echinoderm, contraction being effected by longitudinal muscles, and 

 expansion by fluid jaressure of its contents. It is impossible to avoid being struck by the fact 

 that two of the principal sets of longitudinal muscles, namely those of the metasome and those 

 of the collar, terminate in close connexion with the collar-canals. The probable action of these 

 muscles on the collar-canals has been considered in describing those organs (p. 43). 



The remainder of the muscles of Cephalodiscus appear to be associated with the coelomic 

 epithelium, either of the body-wall or of the alimentary canal. The proboscis of C dodecalophus 

 is provided with a well developed layer of muscular fibres on its posterior wall (Fig. 151), the 

 direction of which appears to differ according to the part of the proboscis which is examined. 



