1204 



TUNICATA. 



referred to, by producing, and being traversed 

 by, the ciliary currents that bring the food to 

 the oesophageal aperture ; and that the ten- 

 tacula of Bryozoa are essentially members of 

 the alimentary apparatus is shown by the fact, 

 that animalcules, &c. are frequently caught 

 and detained by the action of the tentacles, 

 one or more of which, and sometimes even 

 the whole, bend suddenly inwards, and secure 

 such particles as come within their reach, 

 thus taking the character of prehensile labial 

 or oral appendages. * 



The cavity of the branchial sac is often the 

 habitation of parasitical Entomostraca. 



The buccal or branchial orifice of the Asci- 

 dians terminates, as we have already noticed, 

 by a valvular opening in the large delicate 

 membranous sac, which in some respects ap- 

 pears to be a kind of crop, and in others a 

 respiratory cavity. Opposite to it, and in the 

 lower part of this cavity, is the commence- 

 ment of the oesophagus (fig. 778. d), which 

 leads to the stomach and thence to the in- 

 testine. The viscera are always more or less 



Fig. 778. 



Anatomy of Dendrodoa glandaria, magnified about 3 times. (After MacLeay.) 



a, summit of the test open and thrown back ; b, upper part of the branchial sac opened and thrown 

 back with the test, so as to expose its inner surface and the circlet of tentacles surrounding the 

 inside of the branchial orifice ; c, the ventral sinus ; d, oesophageal aperture ; e, stomach ; f, intestine ; 

 g. anus; h, inner aspect of the external anal orifice; i, nerve-ganglion, situate between the two external 

 orifices, and cut through in opening the animal ; j, part of the test or external envelope ; k, k, branched 

 ovary, single, and on the left side. 



lateral, except in Chelyosoma, where they are 

 disposed flatwise, below the branchial sac. 

 The convolutions of the intestine are placed 

 between the respiratory sac and the muscular 

 envelope or mantle, and are either unattached, 

 except by slight filamentous processes arising 

 from the external surface of the branchial sac, 

 or buried in the substance of the liver and 

 ovaries. 



* Mr. A. Hancock, on the Anatomy of the Fresh- 

 water Bryozoa, Annals and Mag. Nat. History, 

 2d series, vol. v. p. 176. See also Lister and Farre, 

 loc. cit. 



The mouth, or aesophageal orifice, is at the 

 base of the branchial sac ; so that when the 

 latter extends to the bottom of the cavity of 

 the mantle, the mouth is also at the base of 

 the mantle. When the branchial sac stops 

 short at the middle of the muscular sac, or 

 when it is bent upon itselfj the mouth also is 

 near the middle. In the species that have an 

 oblong, transverse cavity, the mouth is at the 

 postero-inferior angle of the branchial sac. 

 The mouth is either a simple round hole or a 

 slit, or it is an aperture divided up by slight 

 folds or ridges. It is always destitute of lips 



