TUNICATA. 279 



been doubted whether the Ascidiae, in contracting- their 

 tunics, expel the water through their anal as well as through 

 their branchial orifices. I have distinctly seen this species 

 (Ascidia prunum), as well as others (in particular the A. in- 

 testinalis), propel currents of water through both orifices at 

 every contraction of the tunics ; that from the anal orifice 

 being almost as strong as the one from the mouth of the 

 branchial sac." * But, indeed, long before this, and even 

 previously to the publication of Cuvier's memoir, Carus had 

 detected " a lateral opening furnished with valves," in the 

 sac, by which the water might have egression, and which, 

 says this most ingenious anatomist, satisfactorily explained 

 how these animals have "the power of rejecting the respired 

 water not only through the mouth, but also through the 

 anus." f I believe there is an error in this anatomical 

 explanation. Mr. Garner, who knew well that the water 

 entered the body by both orifices, says, that by one it " en- 

 ters the respiratory sac, and by the other it is drawn into 

 the external meshes of the branchiae. The water drawn in 

 by each opening must make its exit by the same. Those 

 writers who say the contrary must be incorrect, unless the 

 water pass through the stomach and intestine." In like 

 manner the water enters the respiratory organs of some 

 bivalves {e.g. Teredo, Pholas, Mya, and Solen) by both 

 siphons." \ 



The branchial cavity itself is a large flattened sac, which 

 varies greatly in respect of extent, depth, and form. Some- 

 times, as in Ascidia clavata, it occupies only a small portion 

 of the length of the body; oftener, as in Ascidia microcos- 



"If, when all the water is thus brought out of the Ascidia, the animal be im- 

 mersed in that contained in a vessel, in such a manner, that only one of the 

 apertures be under water, it will fill itself completely by that, whether it be 

 the upper or the lower. It is evident from this, that there is a communica- 

 tion between the two apertures ; of which I also had another proof equally 

 demonstrative, in the air which issued from the lesser aperture, and which, by 

 means of a small tube, I could, without force, cause to pass into the greater, 

 and vice versa. When, besides, I kept one of the apertures closed while I 

 blew into the other, the animal swelled like a bag, and the air found no vent. 



" It appears therefore certain, that the upper aperture is the mouth of the 

 animal, and the lower the anus. In fact, by the latter, I have frequently seen 

 the Ascidise discharge matters which had all the appearance of being excre- 

 mentitious. This lower orifice, likewise, communicates with another channel, 

 or organ, as we shall see presently." — T? t avels in the Two Sicilies, iv. 264-6. 



The species on which Spallanzani made his observations is defined : — 

 "Ascidia coriacea laevis subdiaphana, apertura superiorc octagona, humiliore 

 heptagona." — p. 274, pi. 10, fig. 1-7. I believe it to be the As. prunum of 

 Linnaeus. It is not figured by Forbes and Hanley, yet I feci certain that it 

 is a British species. * Edinb. Phil. Journ.j ( >ctob< r, L830, p. 240. 



f Com. Anat., ii. 146, trans. J Mag. Nat. Hist. n. s. iii. 170. 



