386 



TUNICATA. 



portion of the lateral mesoderm-bands (ins) which also give off into 

 the ectodermal vesicle from their posterior ends elements that change 

 into blood-corpuscles. An ectodermal invagination can also be seen 

 forming ventrally (p) and from this is derived the whole intestine 

 of the " nurse " form. This invagination first gives rise by its dilata- 

 tion to the pharyngeal cavity (Fig. 179 B, p), while the intestine 

 proper is derived from a solid cone of cells which develops at the 

 base of the invagination. This cone very soon develops a lumen at 

 first closed at both ends, and this becomes differentiated into the 

 oesophagus, the stomach and the intestine, the rudiment of the 

 -digestive gland also becoming visible. The rudiment of the intestine 

 opens only later into the atrial cavity (cl). The latter develops later 

 than the pharyngeal cavity from an independent dorsal ectodermal 

 invagination (Fig. 181, cl) which, as it enlarges, comes into close 

 contact with the posterior wall of the pharynx. In this way the 

 transverse and somewhat diagonally-placed branchial lamella arises, 

 in which the four pairs of gill-clefts found in this generation (Fig. 

 245, p. 475) soon appear in the form of small round perforations. 

 According to ULJANIN, the two pairs that lie dorsally develop before 

 those that lie ventrally. 



JTl' 



FIG. 181. Dorsal region of an older larva of Doliolum M tiller i (after ULJAMN). d, 

 atrium ; fl, ciliated pit ; m, muscle-hoops ; n, ganglion ; nb, branchial nerve. 



Only the middle part of the rudiment of the central nervous system 

 (Fig. 181, n) retains its original massive character, while the anterior 

 and posterior ends soon become narrower In the anterior narrowed 

 portion, an irregular cavity develops and breaks through into the 

 pharynx. At this point, the ciliated pit (fi) appears and a delicate 

 canal connects it with the sub-ganglionic body (the homologue of 

 the " glande hypophysaire" of the Ascidians). From the middle 

 swelling of the neural rudiment, the actual ganglion and the sub- 

 ganglionic body develop, while the posterior narrowed portion gives 

 rise to an unpaired nerve which runs backwards (rwvus branchialis, nl, 



