ORGANIZATION OF SIMPLE ASCIDIANS. 447 



" salientia viscera possis 



Et perlucentes numerare in pectore fibras." 



The effect upon the eye is that of delicately-toothed 

 oval wheels revolving continually from left to right ; 

 but the cilia themselves are very much closer than the 

 apparent teeth, the illusion being caused by a fanning 

 motion transmitted along the ciliary lines, producing 

 the appearance of waves, each wave representing a 

 tooth of the supposed wheel. 



Whatever little substances, alive or inanimate, the 

 entering water brings into the branchial sac, if not 

 rejected as unsuitable, lodge somewhere on the respi- 

 ratory surface, along which each particle travels hori- 

 zontally, with a steady, slow course, to the front of 

 the cavity, where it reaches a downward stream of 

 similar materials, which hurries onward, receiving 

 accessions from both sides, until at last the whole is 

 brought into the gaping mouth. 



The food of these creatures, indeed, appears to 

 consist entirely of vegetable organisms, principally 

 Desmidia, Diatomacece, and other microscopic forms 

 which abound in their native element, and are con- 

 stantly poured in shoals down their throats by the 

 amazing machinery we have just been describing. 



Impassive and rudely shaped as the Ascidians seem, 

 a close examination shows that some of them are by 

 no means destitute of instruments of sensation ; and 

 to some species the possession of eyes has been attri- 

 buted by observers of well-established reputation ; 

 these consist of six or eight red specks arranged 

 round each of the two external orifices of the sac-like 

 envelope, and which are stated by some anatomists to 



