40 



ASCIDIANS 



BrA 



particles will be seen to converge to the branchial aperture and 

 be sucked in by the inhalent current entering the body. After 



a short interval a certain proportion 

 of the particles will be shot out from 

 the atrial aperture with the exhalent 

 current. 



These particles have passed through 

 the pharyngeal portion of the ali- 

 mentary canal and the cloacal passages, 

 with the water used in respiration, 

 but a considerable amount of such 

 particles taken in with the water 

 do not reappear, as they are retained 

 by the nutritive organs and pass along 

 the remainder of the alimentary 

 canal with the food. The current 

 of water passing in at the branchial 

 and out at the atrial aperture is of 

 primary importance in the life of 

 /2. ^^^ the Ascidian. Besides serving for 

 respiratory purposes it conveys all 

 the food into the body and removes 

 waste matters both intestinal and 



Fig. 15. — Ascidia mentula Linn. , 



froni the right side (natural renal, and aiso expels the reproduc- 



size), Loch Fyne, N B. ; JJr ^.-^^ proc^^ets from the body. 



Branchial apertnre ; At, atrial ^ -^ 



aperture. Arrows show the The TeSt. The tCSt is notable 



direction of the water currents, ^mongst animal Structures for con- 

 taining " tunicine," a substance which appears to be identical 

 in composition, and in behaviour under treatment with various 

 reagents, with cellulose. It is cartilaginous in appearance and 

 consistency, and to some extent in structure, as it consists 

 of a clear (or in some cases fibrillated) matrix in which are 

 embedded many corpuscles or cells. It is the matrix that 

 contains the cellulose, which may form over sixty per cent by 

 weight of the entire test. As the test is morphologically a 

 cuticle, being a secretion on the outer surface of the ectoderm 

 (Fig. 16, ec), the cells it contains have immigrated to it from the 

 body, and it has recently been shown that many of these are 

 mesodermal cells (leucocytes or connective tissue wandering cells, 

 amoebocytes, and in some cases embryonic " kalymmocytes," or 



