92 



TUNICATA. 



A 



tail, which by its undulating movements propels the body 



forward. 



In order to understand the structure of the Ascidians, it will be 

 well to start from these simply organised forms. The most striking 

 character of the Appendicularia, next to the possession of the 

 ventrally-placed swimming tail with its notochord-like skeletal axis 

 (urochord), consists in the absence of a cloacal chamber for the 

 reception of the excreta. The anus is placed in the middle line of the 

 ventral surface ; further, there are two funnel-shaped atrial canals 

 which begin on either side with a strongly-ciliated opening 



into the pharyngeal sac, and open 

 to the exterior right and left, 

 visually rather in front of the 

 anus. These branchial passages 

 arise as invaginations of the ecto- 

 derm, which come into connection 

 with corresponding evaginations 

 of the pharyngeal sac. The in- 

 troduction of nourishment is 

 regulated by two ciliated arcs, 

 which begin at the front end of 

 a short endostyle, surround the 

 entrance of the pharyngeal sac, 

 and run obliquely towards the 

 dorsal surface, where they unite 

 to form a median row of cilia 

 (composed of two rows of ciliated 



Cells). The latter passes back 



fc o the opening of the O3SOphagUS, 



, .,. , 



opposite a narrow ventral ciliated 

 band, which begins at the hind end of the endostyle (fig. 562). 



The Ascidian larvae (PhaUusia) also have, as Krohn long ago 

 discovered, two branchial slits with corresponding atrial passages. 

 The latter, according to Kowalevski, arise as invaginations of the 

 ectoderm, and later on unite on the dorsal side, and then open by a 

 common cloacal orifice. The ectodermal lining of the atrial cavity, 

 which grows round the sides of the pharyngeal sac, consists, therefore, 

 of a branchial layer which is applied to the pharynx, and a parietal 

 layer which forms the internal lining of the outer wall of the peri- 

 branchial or atrial cavity. The atrial cavity extends round the 

 pharynx as far as the sides of the endostyle. The wall of the 



Pio. 561BotryH.w molaceia (after M. Ed- 

 wards). 0, Mouth; A, common cloacal 

 opening of a group of individuals. 



