98 TUNICATE. 



the tail dorsal to the notochord. The tail, thus developed, becomes 

 bent and applies itself to the side of the body opposite to that on 

 which the nervous system is placed (Fig. 564e). Subsequently the 

 skin begins to thicken at the anterior end and gives rise to three 

 papillae, the future papillae for attachment. The rudiment of the 

 nervous system, on which two pigment spots provided with refractive 

 organs make their appearance (eye and auditory organ, fig. 564e,/), 

 is converted at its anterior extremity into a vesicle and is continued 

 above the chorda into the tail (as a cord with a central canal) (A. 

 canina). 



The branchial sac, still closed and formed of columnar epithelium, 

 lies close to the nervous system : it is separated from the ventral 

 wall of the body by roundish uncoloured cells, which are probably 

 the formative elements of the blood and of the wall of the heart. It 

 has at this period the position and relative size of the future pharynx 

 and its posterior dorsal extremity grows out to form the, at first caecal, 

 rudiment of the digestive canal (fig. 564 e, Z)). The mouth is formed 

 from an invagination of ectoderm on the dorsal surface immediately 

 in front of the anterior end of the cerebral vesicle (fig. 564 e, 0). 

 The cloaca first appears as a pair of dorsally-placed epiblastic involu- 

 tions (fig. 564 e, Kl) : these ingrowths meet and fuse with the wall of 

 the branchial sac so that two perforations are formed. The embryo 

 surrounded by the mantle (formed of gelatinous substance with 

 amoeboid test-cells which have wandered into it) now breaks through 

 the villous egg-membrane and passes into the stage of the free- 

 swimming larva, which presents on the right side of the endostyle 

 the first rudiments of the heart, and possesses all the organs of the 

 later Ascidian except the vessels and the generative glands : in its 

 subsequent development, however, it has to go through a decidedly 

 retrogressive metamorphosis. After the larva has attached itself by 

 means of its papillae, the tail aborts, the muscles and notochordal 

 sheath degenerate, and the axial string of the notochord contracts. 

 The nervous system with the pigment organs degenerates, and the 

 cavity in it disappears; the branchial sac, on the contrary, increases in 

 size, and the oesophagus, stomach, and intestine proper become more 

 sharply distinct. The mantle then becomes firm, the mouth opening 

 perforates the gelatinous covering and becomes the entrance to the 

 branchial sac ; behind the mouth the arch of cilia appears at the 

 anterior end of the ventral furrow, which was formed at an earlier 

 stage and gives rise to the endostyle. The opening to the oesophagus 

 becomes funnel-shaped and more distinct. The first branchial slits 



