68 



Some of these fibres may be seen to leave the posterior wall from place to place, and to pass 

 across the body-cavity to the anterior wall, where they are inserted into the basement-membrane. 

 It appears to me that these muscles really form part of the system of radiating muscles which 

 have already been noticed. The system originates from sejjtum '/j, and while some of the 

 fibres pass directly across the body-cavity to the opposite wall, others apply themselves closely 

 to the posterior wall, along which they run for a certain distance, the fibres detaching themselves 

 one by one and crossing the body-cavity to reach the anterior wall. The muscles under 

 consideration accordingly radiate in all directions from the proboscis-stalk. Thus in a sagittal 

 section of the probo.scis pas.sing at some distance from the median plane, the fibres near the 

 middle of the posterior wall might be expected to be cut transversely, while they should become 

 more and more oblique in approaching the two ends of the section. This appears to me to 

 correspond with observed facts. I do not think that the anterior wall of the proboscis has any 

 musculature independent of the fibres which reach it horn the posterior wall. 



The study of the buds (cf. PI. XIII, figs. 176, 179, 181) seems to indicate that the 

 morphologically anterior end of the probo.scis is somewhere near the middle of the thick glandular 

 epidermis of what I have termed the anterior side. If this he the case, the flattening of the organ 

 is in an antero-po.sterior direction, and the radiating muscles may be described, in morphological 

 terms, as a system of longitudinal muscles originating in the posterior half of the proboscis, 

 and inserted into its anterior wall. 



The collar is traversed, in most of its pans, by numerous fine fibrils, running as a rule 

 perpendicularly across the body-cavity from one epithelium to the other. Each fibril is obviously 

 nucleated at about the middle of its course (fig. 151). These fibrils are abundant in the operculum 

 and along the whole length of the arms, e.xcept in the swollen tips which are characteristic of 

 C. dodecalopkus. It appears to me probable that they are contractile, and that the}- are responsible 

 for most of the local alterations of fluid pressure which are so ini])ortant for the effective working 

 of the entire collar-.system. 



In addition to these fibrils, the collar contains numerous more definite muscle-fibres which 

 form part of its body-wall. The principal group is constituted by the strong oral muscles. These 

 have been partially described above, in discussing the functions they appear to perform in the 

 movements of the collar-canals and in continuing the line of action of the body-musculature as 

 far as the base of the proboscis. They are mentioned by M.\.sri.RM.\N (97, 2, p. 353, PI. XXV, 

 fig- 'S) who states that in the mesodermic sheath of the pleurochords originate strong muscular 

 bands which are inserted into septum '/- opposite the origin of the radiating muscles of the 

 probo.scis. This, I think, is only part of the truth, the real facts being that the muscular band 

 in question (on each side) is constituted by factors derived from various parts of the coelomic 

 wall; and that while some of the fibres orisfinate from the basement-membrane coveringf the 

 mouth or pharynx, others start from septum -/:, and others again from the body-wall. Moreover 

 it appears to me that the fibres which originate from the wall of the alimentary canal do not 

 start from the pleurochords, but from a part of the basemenl-nu-mbrane which covers the ordinary 

 epithelium lining the mouth and the commencement of the pharynx. 



r^'to- 93 (^""l- \TH) is a more or le.ss .sagittal section ])assing through one side of a neuter 



