242 Natural History of Molluscous Animals : — 



by more than one competent observer; and if I may not 

 rank myself among these, yet have I witnessed the fact in the 

 Ascidia prunum. You will not dispute the claim of our 

 friend Dr. Coldstream to be numbered among them. " It 

 has been doubted," he says, " whether the Ascidise, in con- 

 tracting 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 par- 

 ticular the A. intestinalis), 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." (Edin. Phil, Journ., October 1830, 

 p. 240.) 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." (Comp. 

 Anat. 9 vol. ii. p. 146. trans.) 



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 microcosmus, 

 it occupies all the length and the breadth of one of the sides, 

 and the rest of the viscera occupies the other side : then its 

 form is oblong, oval, or rectangular. Sometimes, as in As- 

 cidia mammillata and monachus, after having descended even 

 to the bottom of the outer tunic, it bends upwards until its 

 base is at the middle of its length, and looks towards the 

 entrance. In the latter case, the parietes have the greatest 

 extent. In general these are smooth, and without plaits ; but 

 in some species, and, as it would appear, in all those which 

 have a coriaceous outer cloak, they are creased into deep 

 and regular folds, the first vestiges of the four branchial leaves 

 of bivalves.* 



* So says Cuvier, and there is no man whose opinions I so much value 

 and respect ; yet I must acknowledge that there appears to me to be no 

 relationship whatever, either in this or any other genus, between the bran- 

 chial sac of the Mollusca tunicata and the gills of the bivalves. In the 

 former, the water enters the sac by an orifice common to it and the 

 stomach ; in the latter, it reaches the gills by a large fissure, or by a tube 

 always distinct from the mouth : in the former, the aerating vessels are 

 spread on the sides of a portion of the alimentary canal ; in the latter, they 

 form free and independent organs : in the one, they are strictly internal ; in 

 the other, they are properly exterior to the body. Lamarck even considers 



