22 PHYLUM TUNICATA (UROCHORDA) . 



The anterior part of the enteron dilates and constitutes the 

 rudiment of the pharynx (Fig. 15, Ph), from the hind end of which 

 the intestine is developed as an outgrowth (D). In the further 

 course of development the tail becomes greatly elongated and 

 curved ventrally on the trunk (Fig. 15 e), and some ectodermal 

 papillae are formed at the anterior end of the body for the 

 future attachment of the larva (Hp). The anterior end of the 

 medullary tube becomes dilated into the cerebral vesicle, in the 

 wall of which two sensory structures the eye and auditory 

 organ are developed (Fig. 15 /, Gb). The part of the medullary 

 tube immediately behind this acquires thickened walls and is 

 called the trunk ganglion (Rg). This is followed by the narrow 

 caudal extension of the tube which ultimately disappears. The 

 mouth is formed as a perforation on the dorsal surface of the 

 front end and the cerebral vesicle acquires an. opening into the 

 anterior part of the alimentary canal (Fig. 15 /, o). The atrium 

 arises as two dorso-lateral invaginations of ectoderm (kl) into the 

 left of which the anus opens. The at rial invaginations spread 

 laterally round the pharynx but remain separate ventrally ; 

 dorsally they coalesce, so that the single atrial aperture arises. 

 The gill-slits or branchial stigmata arise as a pair of perforations 

 of the wall separating, the atrial cavity from the pharynx. They 

 subsequently become more numerous, partly by formation of new 

 perforations and partly by division of those already existing. 

 The endostyle arises as a groove on the anterior (Willey) or 

 antero-ventral wall of the pharynx, but subsequently, as a con- 

 sequence of the rotation which the body undergoes at the meta- 

 morphosis (Fig. 18), becomes entirely ventral. 



The development of the epicardium, which as we have seen 

 above is probably to be regarded as the coelom of the animal, 

 seems to take place in different ways in different forms. In 

 Ciona it arises from the hind end of the pharynx as two diverticula 

 which remain separate throughout life and invest the digestive 

 viscera like a perivisceral cavity. In Clavelina (Fig. 17, ep) it 

 appears to arise as a single diverticulum of the pharynx between 

 the end of the endostyle and the oesophagus, the front end of 

 which becomes double. In some cases however it is apparently 

 delaminated from the pharynx and is at first solid. The peri- 

 cardium in a great number of cases, if not universally, is nipped 

 off from the epicardium either from'its posterior unpaired portion 



