TUNICATA. 5 



doras had for him great attractions. The Ascidia (Tf^dvu) 

 was one of the many creatures which he examined, in the 

 hopes of gaining definite information respecting such dis- 

 tinction. Its inert and sponge-hke form, rooted to the 

 ground, seemed to indicate a vegetable nature; but Aristotle 

 was not content with a mere external survey — he explored 

 its internal structure, and soon perceived its highly animal 

 condition. His description of the Trjduoi is wonderfully 

 correct : it occurs in the fourth book of his " History of 

 Animals." There he distinctly recognises the Ascidians to 

 be Mollusca, of which, he says, " they are the only kind 

 whose whole body is enclosed in the shell, and that shell of 

 a substance between true shell and leather : it may be cut 

 like dry leather." What comparison could be more graphic 

 or more true ? " They are attached to rocks by their shell. 

 They have two separate openings, which are very small and 

 difficult to notice, the one to take in, the other to eject the 

 water.* * * * If we open them, we find a nervous mem- 

 brane lining this leathery case, and fixed to it at two points 

 corresponding to the openings, one of which may be looked 

 upon as the mouth and the other as the vent." And then 

 he makes further remarks on their anatomy. His appre- 

 ciation of the nature of the Ascidians is an interesting 

 proof of the wonderful sagacity and minute observation of 

 the great Father of Natural History. 



It is worthy of remark, that very lately the Ascidians 

 have again played a part in that much-vexed question of 

 the distinction between animals and vegetables. After 

 Aristotle's demonstration of their affinity with ordinary 

 Mollusca, they had escaped being dragged into this very 

 unsatisfactory discussion. As the sciences have progressed 

 they have approximated, and chemistry has been called to 

 the aid of natural history for the solving of this knotty 

 point. The Ascidians have been obliged to submit to a 

 new cross-examination, and with very unexpected results ; 

 for they have shewn in the composition of their tissues an 

 unlooked-for relation with vegetable structures. In 1845, 

 Dr. Schmidt, in a work entitled " Zur vergleichenden 



