TETHYODEA. 



91 



distance from the mouth a circle of usually simple tentacles. On the 

 neural side of the branchial sac is the cloacal cavity which receives 

 not only the water flowing out through the branchial slits, but also 

 the faeces and the generative products. The digestive canal, together 

 with the other viscera, is some- 

 times placed as in all the simple 

 Ascidians rather to the side of 

 the branchial sac or, as in the 

 elongated forms of the compound 

 Ascidians, simply behind the 

 same, and in the latter case FiulJ 

 often occasions a constriction of 

 the body, so that Milne Edwards 

 was able to distinguish a thorax 

 and abdomen, or even a thorax, 

 abdomen and post-abdomen. 



The Ascidians either remain 

 solitary, and then usually attain 

 a considerable size (A. solitaries), 

 or by budding and throwing 

 out root-processes they produce 

 branched colonies, the individuals 

 of which are connected together 

 by their body walls, and are not 

 embedded in a common mantle 

 covering (A. societies). In other 

 cases (/Synascidice) numerous in- 

 dividuals live in a common man- 

 tle ; they often have a charac- 

 teristic arrangement around a 

 common central opening (A. 

 compositce], so that each group 

 has its central cavity, into which 

 the exhalent (i.e. atrial) openings 

 lead as into a common cloacal 

 cavity (fig. 561). There are 

 solitary (Appendicularia) as well 

 as compound Ascidians (Pyrosoma) which can move freely. The 

 solitary Appendicularice execute the most perfect swimming move- 

 ments. In their external form they resemble the free-swimming 

 Ascidian larvae, and like these they have a whip-like swimming 



FIG. 560. Clavelhna lepadiformis (regne ani- 

 mal), somewhat diagrammatic. 0, Mouth : 

 Br, branchiae ; End, endostyle ; Oe, oesophagus ; 

 G, nervous centre ; MD, stomach ; Kl, cloacal 

 chamber ; A, exhaleut pore ; Af, anus ; GD, 

 genital gland ; Gg, duct of genital gland ; 

 $f, stolons. 



