NOTOCHORD. PHARYNX. 75 



in the stalk and opens into the buccal cavity and the wider 

 head which projects into the proboscis-cavity and in which the 

 lumen may be much reduced and not easy to see. At the junction 

 of the two there is a ventrally directed csecaldiverticulum (Figs. 

 60, 64). The walls of this structure are formed of endoderm 

 which is continuous with the epithelium lining the buccal cavity 

 and consists of cells which have undergone a peculiar modifica- 

 tion, very similar to that of the notochord cells in Amphioxus 

 and in the embryos of the Vertebrata. That is to say, they are 

 much vacuolated and the protoplasm and nucleus occupy but 

 a small portion of them. There is a basement membrane round 

 the whole structure. This, which may be compared to the 

 sheath of the vertebrate notochord. is much developed as a 

 skeletal tissue ventrally and posteriorly (Fig. 60, 19, Fig. 64, n). 

 We shall describe it later on. 



There is considerable variability in the structure of the notochord. In 

 transverse section the neck is often semilunar, but its cavity may be broken 

 up into several separate spaces. This is particularly liable to happen in 

 the neighbourhood of the ventral appendage which in some species is very 

 massively developed. In some species portions of the proboscis skeleton 

 may penetrate the substance of the notochord (Harrimania kupfferi, 

 Balanoglossus biminiensis). In Ralanoglossus carnosus some of the noto- 

 chordal cells lining parts of its lumen, which are quite cut off from the rest, 

 are densely ciliated. In the same species the hind end of the notochord 

 splits up into three minute tubes which unite again before joining the 

 mouth. 



In Stereobalanus canadensis the neck is absent, and the head is therefore 

 quite disconnected from the buccal wall. Speaking generally, it may be 

 said that the notochord behind its ventral diverticulum or caecum, quickly 

 loses its chorda-like character. It is interesting to notice that it is in 

 this posterior portion the skeletal development of the sheath is found. 

 May we draw the inference that anteriorly it has a supporting function, 

 while posteriorly this function is taken over by its sheath ? In Schizo- 

 cardium, Glandiceps, and Spengelia the head is continued forwards as a 

 narrow process which traverses the axis of the proboscis and is either solid 

 or contains a small indistinct lumen. This anterior extension is called the 

 vermiform process of the notochord. The ventral caecum of the noto- 

 L'hord frequently has a considerable lateral extension, whence arise its 

 lateral pouches (e.g. Glossobalanus ruficollis). The lumen of these struc- 

 tures is often cut off from the main lumen. 



The pharynx. The anterior part of the branchiogenital region 

 is occupied by the pharynx, which, as in other Chordata, con- 

 stitutes the branchial * portion of the alimentary canal. The gill- 



: Punnett suggests, as it seems to us with some truth, that the 'unction 

 of the branchial wall with its perforations is more that of a sieve than of 

 a branchial organ, allowing the water which enters by the alimentary 



