1 82 THE ASCIDIANS. 



An average simple Ascidian, as Plialliisia or Cynthia, 

 has been aptly compared to a leather bottle provided with 

 two spouts. The spouts occur in the form of two funnel- 

 like prominences projecting from the surface of the body 

 and bearing at their free extremities the inciirrent or buc- 

 cal and excitrrent or cloacal apertures respectively, the 

 latter usually occurring at a lower level than the former. 



The most prominent and, apart from the two apertures, 

 the only external feature of a simple Ascidian, is the char- 

 acteristic tunic or test which surrounds the whole body. As 

 a rule, all Ascidians of whatever kind possess this external 

 tunic, and it is one of their chief diagnostic characters. 



According to the species this test may be of a cartilagi- 

 nous, coriaceous, fibrous, or membranous consistency, 

 usually opaque, but sometimes hyaline and transparent, as 

 in Corella, Salpa, etc. Its outer surface may be smooth, 

 wrinkled, or rough, capillated, papillated, or mammillated. 

 In 1845 Karl Schmidt made the discovery that the test 

 of the Ascidians was largely composed of the substance 

 which forms the cell-walls in plant tissues ; namely, cellu- 

 lose. When treated with the proper chemical reagents, it 

 gives the cellulose-reaction. This is interesting as show- 

 ing the fundamental identity of protoplasm whether it 

 occurs in animal- or in plant-cells, since in both cases it 

 is capable of depositing cellulose. 



Judging by external appearances an ordinary Ascidian 

 resembles nothing so little as Amphioxus, and yet it is 

 probably more closely related to the latter than is the 

 lamprey larva, Ammoccetes, whose external resemblance 

 to Amphioxus is incomparably greater. 



It is only in its internal organisation that we meet with 

 structures which remind us strongly of corresponding- 

 parts in Amphioxus. 



