38 



PHYLUM TUNICATA (UROCHORDA). 



endodermal vesicle becomes constricted into two parts which remain 

 connected by a narrow neck ; the upper of these gives rise to the pharynx 

 and atrial cavity, and the lower to the epicardium from which the 

 pericardium subsequently proceeds. The digestive tube arises from the 

 pharynx in the ordinary way, while the central nervous system appears 

 to develop from the pharyngealendoderrn,* as it does also in the buds of 



Distaplia t and Botryllus i and 

 probably in those of all Ascidiae 

 Compositae. It would thus appear 

 that in the Ascidiae Compositae the 

 atrial cavity and the central nervous 

 system,which in the embryo develop 

 from the ectoderm, in the bud de- 

 velop from the endoderm. This 

 contrast is highly remarkable, and 

 due weight will have to be given 

 to it, when we are considering in 

 the General Part, the theory of the 

 embryonic layers. 



Clavelina Sav., stolons delicate 

 and branched, body divided into 

 thorax and abdomen, without pe- 

 duncle, pharynx without inner 

 longitudinal bars ; 2-3 cm. in 

 length ; Eur. and Med. Podocla- 

 vella Herd., with peduncle, Arctic, 

 Australia. Stereoclavella Herd., 

 stolons and hind end of body in a 

 common mass of test, Atlantic, 

 Australia. Pycnoclavella Garstang, 

 (J.M.B.A. (2), 2, 1891), similar to 

 preceding, Plymouth. Perophora 

 Lister, without abdomen, short 

 peduncle, pharynx with 4 rows of 

 stigmata, transverse bars of pharynx 

 with papillae which branch and may 

 form imperfect internal longitudi- 

 nal bars (Fig. 8), zooids 3-5 mm. ; 

 Atl., Med., N. Amer., Austr. Pero- 

 phoropsis Lahille, with 15 or 16 

 rows of stigmata, Med. Ecteina- 

 scidia Herd., without abdomen, 

 test without blood-vessels, pharynx 

 with internal longitudinal bars, 

 most seas. Sluiteria v. Ben. Dia- 

 zona Sav., many zooids united by 

 tests posteriorly to form a large colony, abdomen present and embedded 

 in common test, pharynx with internal longitudinal bars, zooids 3-6 cm. 



FIG. 31 . Portion of a proliferating stolon 



b of Perophora (after Kowalevsky from Kor- 



schelt and Heider). ec ectoderm; en en- 



doder m ; kn buds ; s stolonic septum ; 



^branching of the stolon. 



* Seeliger, Z.f.w.Z., 56, 1893, p. 365-401. Ritter, Journ. Morph., 12, 

 1896. 



t Kowalevsky, Arch. Mik. Anat., 10, 1874. 

 J Hjort, ZooL Anz., 16, 1892, p. 328. 



