70 



the fibres of the oral surface run parallel to the free edge (fig. 148), while those of the outer 

 wall cross them at right angles. The explanation of the course of the opercular muscles seems 

 to me, however, much the same as that given for the proboscis; namely that the muscles are 

 really a system of fibres which radiate out from the centre of the collar, the course of the 

 fibres being differently modified on the two surfaces. 



Thus the fibres of the oral side of the operculum sweep round the free border of the 

 lateral lobe in a direction parallel to its free edge (fig. 148), while those of the aboral surface 

 pass transversely outwards along the same region (fig. 149), and thus cross the other fibres 

 at right angles. The extreme mobility of the operculum, of which there is excellent evidence in 

 the varying j^ositions assumed by this organ in preserved specimens, is doubtless due to the 

 complicated course of the fibres of the musculature of its body-wall. 



In accordance with what has already been said in a previous Section (p. 30) with regard 

 to the morphology of the collar, both the operculum and the arms may be considered to be 

 modifications of the free anterior edge of that region. The musculature of the arms is thus 

 probably to be regarded as longitudinal. I am inclined to think that that of the operculum has 

 really the same morphological direction, and that the actual course is the result of modifications 

 of a longitudinal layer radiating out into what is practically a funnel-shaped edge formed by 

 the arm-bases and the operculum. It is indeed not easy to explain the circularly disposed muscles 

 seen in fig. 107 as longitudinal, since they appear to be in the line of the transverse circumference 

 of the collar; but it must specially be noted that they seem to form part of the system of fibres 

 seen on septum 7.i- The main parts of the oral muscles again appear to have a circular or 

 transverse direction, but their longitudinal character is perhaps indicated by the fact that some 

 of their fibres pass from septum 'j-j through the coUar-cavity to septum V^- I'i spite of these 

 difïiculties of interpretation, I am inclined to take the view, although with a good deal of 

 hesitation, that the system of collar-muscles of Cephalodiscus is morphologically longitudinal. 

 Some of the fibres can hardly be explained in any other way, and moreover I have been unable 

 to obtain evidence that more than one layer is present at any point. It may thus possibly be 

 the case that the entire somatic muscular system of Ccphalodiscits is longitudinal. 



In the arm-bases, there is a considerable development of muscular fibres in connexion 

 with both the anterior and the posterior wall, although all the fibres are probablj- morphologically 

 loneitudinal. There is clear evidence that numerous fibres run across the coelom from one wall 

 to the other, as represented in figs. 147 — 149. These fibres have definitely the character of 

 muscle-fibres, as judged by their staining properties, and they appear to differ histologically 

 from the delicate nucleated filaments which cross the cavity of the operculum. At the origin 

 of the arms, the fibres have a complicated course, and there is a good deal of crossing of the 

 fibres which traverse the collar-cavity in different directions. 



The arms themselves (fig. 141, 5, 6) are provided with a strong layer of longitudinal 

 muscular fibres, on each side of the middle line, runing along the whole of the anterior, grooved 

 surface, and with a similar, but somewhat weaker, layer on each side of their dorsal surface, 

 continuous with the muscles of the dorsal or posterior wall of the arm-bases. 



