24 



The differences between what I regard as the more primitive condition shewn by the 

 bud and the adult arrangement are in reahty very shght, and can easily be accounted for by 

 differential growth. The outlines of the text-figure are taken from a camera lucida drawing, 

 the scale of which is indicated. Comparing this with fig. 42 it will be seen that while the 

 diameter of the stalk of the bud, at its base, slightly exceeds 150 u.., that of the adult stalk, 

 in the same relative position, is less than 200 a., in spite of the thickening which it has no 

 doubt experienced as the result of the contraction of its longitudinal muscles. There is thus but 

 little increase in the thickness of the stalk of the adult, as compared with that of the bud, 

 the principal increase in size of which has been due to an elongation of the metasome along 

 an axis which is indicated in the bud by a line passing through the middle of the pharynx, 

 oesophagus and part of the second stomach. This has resulted in the separation of the base 

 of the stalk from the mouth, and in the outgrowth of the caecum of the metasomatic wall 

 containing the bend of the alimentary canal. With this growth has been associated a ventral 

 flexure of the pharynx just behind the mouth ; a strongly developed dorsal groove which is 

 present in this region remaining to mark the point at which flexure took place as the dorsal 

 diverticulum (fig. 42, div.) of the pharynx. It thus follows that while there is hardly any 

 distortion of the primitive symmetry in front of the line indicating the transverse plane in the 

 text-figure, and comparatively little on the dorsal side of the horizontal line indicating the 

 frontal plane, the principal modification of the axes of the adult body is due to alterations in 

 the lower left quadrant of the text-figure. The pharyngeal limb of the alimentary canal has to 

 a large extent lost its antero-posterior direction, and has assumed a position nearly parallel to 

 that of the intestinal limb. 



The correctness of this orientation is confirmed by the consideration of the mesenteries 

 of the metasome. The dorsal mesentery is confined to the region between the two limbs of 

 the alimentary canal, while the ventral mesentery extends into the stalk, carrying with it what 

 may for the present be termed a loop of the ventral vessel. Indications of the anterior (a. v.) 

 and posterior {/>. v.) limbs of this vascular loop are seen in fig. 42. 



A further confirmation of the orientation here adopted is afforded by a consideration 

 of horizontal sections on the dorsal side of the line indicating the frontal plane in the text- 

 figure. A section in this plane of a young bud was figured by me (87, p. 39) some years 

 ao-o. and demonstrates the Balanoglossus-like structure of the animal. But even in the adult 

 zooid, horizontal sections in the same region still shew the coelomic cavities in a Balanoglossus- 

 like arrangement, as is evident from figs. 154, 155 (PI. XII), which though not actually fully 

 grown, do not differ from the adult condition in this respect. A transverse section of a young 

 Ccphalodiscns is thus one which passes along the line so marked in the text-figure. It is 

 convenient to describe a corresponding section which passes through the long axis of the adult 

 as "transverse"", althouoh it is obvious from what has been said that it will not cut all the 

 organs of the metasome in a stricth' transverse plane. 



Adopting this view, it may be said that the proboscis is flattened in an antero-posterior 

 direction to form the great buccal disc. The collar forms a ring which would be transverse 

 were it not for the fact that its dorsal region is much more developed than its ventral region. 



