42 



closely packed cells, and it l:)ears long cilia or flagella (fig. 140). In either a transverse or a 

 sagittal section this high epithelium has a characteristic crescentic form. 



The dorsal wall of the canal is formed of a much thinner epithelium (figs. 131, 140; 

 PI. \', figs. 46 — 50). It may either be convex towards the lumen of the canal, and therefore 

 more or less parallel to the ventral wall (figs. 49, 131), or it may be flat (I'l. Xlll, U^r. 185), 

 or it may be concave towards the lumen (figs. 101, 140). The latter condition gives the largest 

 size to the cavity, and the collar-canal may be said to be dilated when it is in this condition. 

 It appears obvious that the dorsal thin wall is capable of a considerable amount of movement, 

 and it may be j^resumed that this mobility is of physiological importance. 



It is further obvious that the longitudinal muscles of the trunk end in the immediate 

 neighbourhood of the collar-canal (fig. 140, 77, 93); that a specially strong band of collar-muscles 

 originates in much the same position (figs. 131, 77, 93, or. »i.)-^ and that the anterior fle.xible 

 wall of the canal is in the closest connexion with a tissue (figs. 131, 140, 121, i2 2,,i-.) which 

 appears to consist of a series of radiating lamellae. 



The above facts can easily be verified in any female or neuter zooid (C. siSogae), although 

 in the males of C. sibogae the collar-canals are so small that the details of their structure have 

 not been made out. 



It follows from the observations of Spencel (93, p. 475) that the collar-canals of Balano- 

 glossus are of special importance in taking water from the outside into the collar-cavities, and 

 so maintaining the collar as a whole in a proper state of turgidity. I think it cannot be doubted 

 that the same is true of CcpJialodiscus. The whole mechanism of the collar appears to depend 

 on the proper maintenance of this condition. The movements of the arms and of the operculum 

 are alike dependent on it. The arms in particular are provided with strong longitudinal muscles, 

 by wliich the)- can be contracted ; but for their elongation and for that of the tentacles, one 

 must look to the fluid pressure of the contents of the collar-cavities. The mechanism is j^robablv 

 identical with that of the tube-feet of an Kchinoderm, and the resemblance is made all the 

 more striking by the fact that most jjarts of the collar-coelom are traversed by short fibres 

 which connect the opposite walls (PI. X, fig. 122) as in the case of the ambulacral canals of 

 a Crinoid, for instance. When it is remembered that MacBride (96, p. 39O) has given reasons 

 for regarding the hydrocoel system of an Echinoderm as actually homologous with the left 

 collar-cavity of Balanoglossus the similarity between these cavities in Cephalodisctis and the 

 ambulacral vessels of an Echinoderm becomes all the more noteworthy. 



A strong contracticjn of the longitudinal muscles of the arms must have the effect of 

 forcing their fluid into other parts of the collar; and as there are no spaces which appear to 

 have the function of reservoirs, I think it follows that fluid must leave the collar b\ way of 

 the collar-pores. During the e.xtension of the arms, on the contrary, fluid must be supposed to 

 be taken in by the collar-pores, which thus act now as exhalant and now as inhalant apertures. 

 How are these varying currents controlled? The strong flagella of the ventral epithelium of 

 the collar-canals are no doubt of special importance in the process; but the mode of ending of 

 some of the strongest muscles of the body in the immediate vicinit\- of the jjores is .significant, 

 and it appears to me that it may imply something in the nature of a pumping action. 



