298 TUNICATA THEIR DIGESTIVE ORGANS. 



stant and quick vibration when the animal was left undis- 

 turbed. I presume them to be organs of a very delicate 

 irritability, perhaps of taste ;* and that their purpose is to 

 hinder the ingress of noxious matters, not altogether mecha- 

 nically, but because the sudden contraction of the oral aper- 

 ture is a necessary sequence of their unpleasant irritations, f 

 This aperture leads directly into the branchial sac, which, 

 besides its office of a respiratory organ, seems to perform 

 in part that also of a stomach ; few that the process of di- 

 gestion commences there, seems obvious from the fact, that 

 numerous animalcules are generally found in it, but are 

 never to be detected in the viscera of the abdomen. At the 

 base of this sac there is another aperture (called by Cuvier 

 the mouth), * which conducts us, through the medium of a 

 narrow membranous tube or oesophagus, into the proper 

 stomach, an organ always much smaller than the branchial 

 sac, very variable in point of situation and form, generally 

 puckered into longitudinal plaits internally, and sometimes 

 studded with some glandular bodies ; but its minute struc- 

 ture cannot be ascertained with any degree of accuracy. It 

 contains, in general, only a little liquid ; J while the intes- 

 tinal canal, on the contrary, is almost, in every instance, 

 filled throughout with a sufficiently consistent matter, some- 

 times grumous, more often homogeneous, of a yellowish grey 

 colour, and rolled into little round or egg-shaped pellets, 

 which it behoves us not to mistake for the proper ova. This 

 canal is usually wide, and has a flexuous course ; at first de- 

 scending in the common sac, and then returning upon itself, 

 it winds along the anterior side of the branchial sac, to open 

 outwardly by a round aperture placed near the mouth, but 

 distinguished by its lesser prominence. In the Alcyonese, it 

 is otherwise like the mouth in form and structure ; but, in 

 Ascidise, there is no filamentous fringe at this orifice ; which 

 is furnished, instead, either with two valvular folds, or with 

 a simple circular plait. 



* " The disposition of the alimentary canal determines, in a manner 

 perfectly absolute, the kind' of food by which the animal is nourished ; but 

 if the animal did not possess, in its senses and organs of motion, the means 

 of distinguishing the kinds of aliment suited to its nature, it is obvious it 

 could not exist." CUVIER, Comp. Anat. i. 55. trans. 



f " II est garni d'une rangee de filamens charnus, ou de tentacules tre"s- 

 fins, qui servent sans doute a 1'am'mal pour 1'avertir des objets nuisables 

 qui pourroient se presenter et qu'il doit repousser." CUVIER, Mem, xx. 

 10. 



J From this circumstance Savigriy infers that the more gross and indi- 

 gestible parts of the food are regurgitated, as they are in some nocturnal 

 birds of prey. Mem. sur les Animaux sans Vert. ii. 8. 



