1202 



TUNICATA. 



it occupies all the length and one of the 

 sides of the cavity of the mantle (Cynthia). 

 In Ascidia mammillata and A. monacha it is 

 very long; and, after having descended to the 

 bottom of the mantle.it is bent upwards upon 

 itself, extending half way up the cavity again. 



A tube, differing in size in various species, 

 which has its analogue, perhaps, in the retrac- 

 tile operculum of the Bryozoa, conducts the 

 water from the buccal aperture to the respira- 

 tory sac. This tube or process of the mantle 

 encloses the neck of the branchial sac. At 

 the inner extremity of the tube, where it 

 widens into, and is continuous with, the bran- 

 chial sac, are frequently observed about five 

 small valvular folds ; and below these there 

 is always a circle of fleshy filaments or 

 tentacular appendages (fig- 778. arid fig. 

 39 d. Vol. I. page 1 12. ANIMAL KINGDOM*). 

 These are present also in the Clavellimdcc 

 and the Botryllidce. They vary from 6 to 

 26 in number, and are either simple, as 

 in Pkallufiia, or branched, as in Cynthia, Bol- 

 tenia, &c. By detaching the anterior part 

 of the respiratory sac of Cynthia, or others, 

 and examining it from its inner and inferior 

 aspect, one may perceive, at the inner mar- 

 gin of the tube, several arboriform append- 

 ages, somewhat large at the base, and having 

 the branches either swollen at their extremi- 

 ties, or finely laciniate and almost plumiform : 

 in Cynthia Dione they are somewhat bipinnate. 

 The number of the tentacles is difficult to be 

 arrived at, as smaller filaments appear amongst 

 the six or seven larger ones. The branches 

 are generally directed downwards, towards 

 the bottom of the sac, but sometimes are 

 slightly curled upwards. Seen under the rni- 

 croscope, these filaments are hollow ; and the 

 extremities and swellings of the branches are 

 cul-de-sacs. The several tentacles intercom- 

 municate with each other by their hollow 

 bases, and with the vascular network of the 

 respiratory sac. 



The walls of these tubular organs are very 

 thin and transparent ; traces of muscular 

 fibre have been detected in their substance, 

 and a circulation ; hue no trace of vibratile 

 cilia, either on their outside or within their 

 cavity. Van Beneden observes that a fluid 

 traverses their cavity in a similar manner to 

 the blood in the branchiae of the Doris. In 

 Actinia and Holothuria simple and ramified 

 filaments occur, that offer considerable analo- 

 gies to the arborescent tentacles of Ascidiae. 

 They are all hollow; and the fluid moving in 

 their interior comes directly from the peri- 

 intestinal cavity ; this great cavity, commu- 

 nicating as in Bryozoa, with the tentacular 

 appendages. 



The internal surface of the respiratory sac 

 is sometimes uniform (in Ascidia, Pha/litsia, 

 &c.), and frequently longitudinally plicated 

 and disposed in deep and regular folds, all 

 following the curve of the cavity, and termi- 



* This figure represents a Cynthia canopus cut open ; 

 a, oesophagus ; b, stomach ; c, anus ; d, external 

 anal orifice, closed ; e, branchial orifice, laid open ; 

 f, branchial tentacles; g, nerve-ganglion; h, dorsal 

 sinus; ft, ovary (?); i, I, indeterminate bodies. 



nating at a little smooth area above the pha- 

 rynx (Cynthia, Boltenia, c.). The folds 

 are from 8 to 18 in number, and form the 

 first indication of the four branchial laminae 

 of the bivalve Acephalans. The structure 

 of the membrane consists of an infinity of 

 small, anastomosing vessels, generally cross- 

 ing each other at right angles, anil forming 

 quadrangular interspaces, which, under the 

 microscope, are seen to be still more mi- 

 nutely subdivided in the same manner. Milne- 

 Edwards observes that in Pka/tusiaeiich of the 

 meshes of the respiratory membrane is occu- 

 pied by a minute spiracle, that allows of a 

 communication between the interior of the 

 branchial sac and the cavity of the mantle 

 (the " thoracic chamber" of Milne-Edwards), 

 the dorsal portion of this chamber being the 

 cloaca, the base of which is occupied by the 

 orifices of the digestive and generative tubes, 

 and the summit of which opens externally by 

 the anal aperture. In Cynthia ampulla the 

 meshes are very irregular and almost inextri- 

 cable, some of the minute vessels having 

 apparently a spiral arrangement ; and the 

 vessels are rather more numerously disposed 

 around the cesophageal orifice. A somewhat 

 spiral or vermicular arrangement of the ex- 

 tremities of the branchial vessels exists also 

 in Chelyosoma, as figured by Eschricht ; * and 

 as seen in a very minute sessile Ascidia, from 

 South America, in the collection of Mr. Bower- 

 bank. In the respiratory network of Cynthia 

 the large longitudinal vessels are seen to be the 

 most prominent of all. They form, with the 

 large transverse vessels, square meshes, which 

 three other shorter vessels subdivide into four 

 transverse meshes ; and these are further 

 intercepted by extremely fine longitudinal 

 vessels. The vertical vessels of this quadran- 

 gular network may be said to arise from the 

 transverse vessels, which communicate by 

 each extremity with two vertical trunks, placed 

 at opposite sides of the sac, and representing 

 respectively the branchial artery and vein. 

 The latter, in C. microcosm us and other As- 

 cidians, bears a longitudinal series of small 

 transverse tentacular filaments. Similar ten- 

 tacles we shall have to describe as belonging 

 to Chelyosoma (fig. 777. i). 



The angles of the meshes of the branchial 



O . 



tissue bear papillae, more or less prominent, 

 in some genera (Ascidia, Chelyosoma, &c.) ; but 

 in others they are not papillated ( Cynthia, 

 &c.). These papillae, or minute pouches, are, 

 according to Savigny, analogous to the fila- 

 ments that border the branchial vein, in most 

 of the simple and compound Ascidians, indi- 

 cating the junction of the transverse vessels 

 with that vein. The appearance of the reticu- 

 lation is large and coarse in Cynthia, minute in 

 Ascidia, and indistinct in Cyxtingia. 



Each mesh of this respiratory network is 

 thickly fringed with vibratile cilia, as in the 

 rest of the Tunicates and Acephalans ; and 

 Van Beneden points out that this ciliated ex- 

 terior of the branchial vessels is analogous 

 to the ciliated tentacles of the Bryozoa. By the 



* Hoy. Danish Transact, vol. ix. pi. 1. fig. 6 & 7. 



