TUNICATA. 



1239 



ANATOMY OF PELONAIA.* Muscular sys- 

 tem. The mantle is similar to that of other 

 Tunicates, possessing longitudinal and circular 

 fibres ; the former in P. corrucala forming a 

 thick bundle at their origin round the respira- 

 tory opening. A strong band of transverse 

 fibres passes round it, in one species, imme- 

 diately below the anal orifice, encroaching on 

 the cavity principally on that side. The chief 

 peculiarity of the mantle is its firm adhesion 

 to the test. 



Fig. 789. 



Anatomy of Pelonaia glabra. (After Forbes and 

 Goodsir.) 



a, a bristle inserted into the respiratory sac 

 through the oral orifice ; b, a bristle inserted into 

 the mantle cavity through the anal orifice of the 

 test ; c, ganglion, with the nerves proceeding from 

 it ; d, the shelf, or transverse ridge, in the interior 

 of the test and mantle ; e, branchial vein, enclosed 

 in a serpentine band, as in some of the other Asci- 

 dians ; f, branchial artery ; g, generative organ on 

 the left side, with a bristle inserted into its duct ; 

 h, the stomach ; i, anus ; , k, cut edge of the test. 



Digestive and respiratory systems. The 

 respiratory opening is of small size, and ex- 

 hibits no folds or tentacular fringes. The 

 respiratory sac is elongated, cylindrical, con- 

 tracting rather suddenly towards one side to 

 become continuous with the oesophagus. On 

 the external surface of the sac there are about 

 thirty parallel transverse ridges, which give it 

 the appearance of a plaited frill. These plaits 

 are less apparent along the course of the 



* From Forbes and Goodsir, foe. cit. 



branchial artery and vein, but midway between 

 them on each side they are very prominent, 

 and are tied, each by a minute cord, to the 

 inner surface of the test. The internal sur- 

 face of the sac exhibits along one side the ser- 

 pentine double cord which contains the bran- 

 chial vein ; along the other side the branchial 

 artery ; and from these primary and secondary 

 perpendicular branches proceed, as in other 

 Tunicata. The transverse plaits on the ex- 

 ternal surface of the sac correspond to the 

 primary or transverse branches of the vessels 

 on the internal surface. During the life of 

 the animal, without doubt, cilia exist in great 

 abundance on the edges of the lozenge-shaped 

 spaces of the sac. 



The oesophagus commences by a white 

 plicated opening at the lower end, and on one 

 side of the sac (Jig. 789.). It is curved in a 

 sigmoid form, and exhibits longitudinal rugae 

 through its coats. Near the lower end of the 

 mantle-cavity it terminates by suddenly dilating 

 into the stomach (A), which is pear-shaped, and 

 directed obliquely upwards towards the side 

 opposite to the oesophagus. The internal 

 surface of the stomach presents longitudinal 

 plica;. It is succeeded by the intestine, which 

 at first curves upward, and then down to the 

 bottom of the branchial cavity, up along the 

 cesophageal side of that cavity and between its 

 walls and the branchial artery, terminating 

 about the anterior third of the animal in a 

 funnel-shaped anus (), which is cut into ten 

 or eleven processes, like the petals of a flower. 

 The first part of the intestine is white, and 

 longitudinally plicated ; the rectum is dilated, 

 and its coats are attenuated. 



Vascular system. The vascular system re- 

 sembles that of the Asddiadce, except that 

 there is no heart. It consists of two sets of 

 vessels, with four sets of capillaries ; a circle 

 in fact twice interrupted, once in the respira- 

 tory sac, and again throughout the body. The 

 branchial veins run along the transverse plaits 

 of the sac, receiving secondary and ternary 

 twigs at right angles. The primary branchial 

 venous branches empty themselves on each 

 side into the branchial trunk, which runs in 

 the substance of the double cord which coasts 

 the superior aspect of the sac. This double 

 cord terminates in an abrupt manner anteri- 

 orly near the oral orifice, and in a similar 

 manner, but often becoming smaller near the 

 orifice leading to the oesophagus. At this 

 point the vein becomes an artery, and pro- 

 bably sends back vessels to nourish the sac. 

 It now runs along the oesophagus, supplying 

 the stomach and intestine, and giving off in 

 its course branches to the mantle. The veins 

 arising from the arterial capillaries of the 

 body meet near the commencement of the 

 oesophagus in one trunk, which, passing along 

 the inferior wall of the respiratory sac, oppo- 

 site to the branchial vein, performs the func- 

 tions of a branchial artery. It is interesting 

 to observe here the differences between the 

 modes in which the branches enter the 

 branchial vein, and strike off from the bran- 

 chial artery. In the former, just before the 

 4x4 



