TUNICATA. 



1219 



excretory orifice, not occupying the centre of 

 the " system," but placed nearly at one of the 

 extremities, and communicating with the in- 

 dividuals situated at the other end of the 

 group by means of the interior canal. 



In Amaroucium the faecal or anal orifices, 

 by which the common cloacae open externally, 

 are wide open during life, and easily per- 

 ceived ; their form is round and their border 

 thick ; but after death they contract, and are 

 so much sunk in that they are demonstrated 

 with difficulty. 



The anal orifice in some genera is sur- 

 mounted by a membranous languet, either 

 simple or trifid (Jig. 7,83. *'). 



Fig. 783. 



Anatomy of Botrylloides rotifera. (After Milne- 

 Edwards.) Lateral view of an isolated individual 

 magnified. 



c, branchial orifice; e, branchial stigmata; /, 

 thoracic sinus ; h, cloaca ; t, anal orifice ; i', supra- 

 anal languet; j, nerve-ganglion; k, oesophagus, 

 Z, stomach ; m, first portion of intestine, or duo- 

 denum; mf, second portion, or chylific ventricle; 

 m", third portion, or rectum ; n, anus, opening into 

 the cloaca ; q, testicle ; r, vas deferens ; t, radiciform 

 appendage, or proliferous stolon; t', reproductive 

 germs ; x, liver ( ?) ; x>, its excretory duct. 



In pointing out the natural groups of the 

 associated Ascidians we have spoken of the 

 division of the body into distinct portions, 

 as the thorax, and super- and post-abdomen 

 in Amaroucium (fig. 782.), and in Polydinum, 

 where they correspond to the three vertical 

 chambers of the animal's cell ; and into the 

 thorax and abdomen in Didemnum. The 

 latter arrangement resembles that of the Cla- 

 veiling; but in the Botryllians, on the other 

 hand, no distinct separation is observable, the 

 viscera being pushed up by the side of the 

 branchial sac, as in the Peroplwrce and the 

 simple Ascidians (fig. 783.). The thorax is 

 generally more or less cylindrical, sometimes 

 hemispherical (fig. 769.) or subglobose, and 

 contains the branchial organs. In Botryllus 



violaccus there are two little glandular tuber- 

 cles, one on the right and one on the left of 

 the buccal orifice, and situated at a nearly 

 equal distance from the superior extremity of 

 the ventral sinus and the dorsal nucleus or 

 nerve-ganglion. 



The branchial sac of the Botryllidce is very 

 similar to that of the Clavellinidce. The bran- 

 chial spiracles, or intervascular spaces, are 

 variable in number, and the crest or fold cor- 

 responding to the anterior border of the 

 branchial sinus has no membranous languet ; 

 but in Diazona, Synoicum, and Polydinum it 

 bears a row of minute tentacular filaments. 

 The branchial sac of Sigi/lina has 4 large, 

 salient, transverse vessels on each side, united 

 by 15 or 16 smaller longitudinal vessels ; 

 and in Aplidium there are 10 to 12 trans- 

 verse branchial vessels. Polydinum has 14 

 transverse branchial vessels anastomosing with 

 15 to 18 finer longitudinal vessels. In Ama- 

 roucium there are 10 to 12 rows of stigmata, 

 and in Polydinum 13 rows. Didemnum and 

 Leptodinum have 5 rows. In Botrylloides 

 the respiratory sac presents 10 vertical rows 

 of spiracles, parcelled into threes by vertical 

 folds. There are 9 of these chink-like open- 

 ings on each lateral rank, and at each of their 

 four angles is a little tubercle. In Botryllus 

 the respiratory sac lies almost horizontally, 

 and has on each side 9 transverse rows of 

 stigmata, grouped into threes by the longi- 

 tudinal folds. The angles of the branchial 

 net-work are marked with papillae in Distoma 

 and Diazona. 



Instead of papillae on the spaces between 

 the branchial meshes, Leptodinum Listen has 

 a thin ledge between each row of spiracles ; 

 and in front there are three tapering moveable 

 prominences, one connected with each ledge, 

 either stretched forward horizontally into the 

 cavity, or bent downwards with a spiral curve. 

 These laminae, observes Mr. Lister, seemed to 

 suspend a generally invisible vertical mem- 

 brane, and to assist in giving the food its 

 direction towards the stomach ; for it moved 

 horizontally along the sides of the cavity, as 

 in Perophora, and when it reached the front, 

 took a spiral motion downwards. When the 

 branchial sac of the Leptodinum contracted 

 forcibly to reject what had been stopped by 

 the tentacles, or found unfit for food, the oral 

 orifice, instead of projecting, was then drawn 

 down below the level of the external test, 

 and depressed it, the cilia being also closely 

 stretched across the openings of the spiracles. 

 When the cilia were thus stopped in their 

 action, they were seen to be very numerous, 

 and being in close contiguity one with another, 

 the neighbours by their sides, and the oppo- 

 sites by their ends, appeared almost as a con- 

 tinuous membrane. 



The superior abdomen of the Polydina 

 contains the digestive apparatus, and the post- 

 abdomen the organs of generation and the 

 heart. The intestines of the BotryUidce are 

 always subject to one or more foldings, and 

 their several portions are frequently distin- 

 guished by different tints of colour. 



4 i 2 



