TUNICATA. 



1215 



sac. The spaces circumscribed by the narrow 

 elongated meshes, supported by the vertical 

 filaments, and bordered with vibratile cilia, 

 are not occupied by a membrane, but form 

 openings, somewhat like a button-hole, called 

 branchial stigmata. These chinks or spiracles, 

 through which the water passes from the 

 branchial sac in the thoracic chamber to es- 

 cape outwards by the anal orifice, are conse- 

 quently disposed in the same manner as the 

 vertical filaments that circumscribe them, that 

 is, parallel to each other and in transverse 

 ranks, thirteen in number. Lastly, each of 

 these vertical filaments is perforated by a 

 canal, opening by its two extremities in other 

 similar but larger canals, that occupy the in- 

 terior of the transverse fillets. The latter 

 vessels in their turn empty themselves by one 

 of their extremities into the great dorsal or 

 branchial sinus, and by the opposite extremity 

 into the vertical fold of the ventral wall of 

 the thoracic chambers, which is circumscribed 

 by two parallel, vertical, yellow lines, and 

 which places the respiratory organs in relation 

 with the great thoracic sinus, lying between 

 this chamber and the ventral portion of the 

 internal tunic of the body. These two sinuses 

 communicate also one with another by the 

 vessels that surround the branchial orifice, 

 and from which spring other vessels that de- 

 scend towards the abdomen. 



The pharyngeal portion of the thoracic 

 chamber, tapestried with the branchial net- 

 work, opens externally at its upper extremity, 

 by the branchial aperture, which occupies 

 nearly the whole of its diameter, and is 

 furnished with a kind of radiating net-work of 

 tentacular filaments. This cavity is cylindrical, 

 and at its inferior extremity has a large trans- 

 verse slit, the opening of the oesophagus, oppo- 

 site to the external oral aperture. The oeso- 

 phagus is large, and descends vertically into the 

 abdomen terminating in the stomach (Jig. 768. /), 

 which is ovoid and swollen. The intestine (wz) 

 springs from the inferior extremity of the 

 stomach, and at first running vertically down- 

 wards, and then bending forwards and upwards, 

 so as to form a loop, it ascends towards the 

 thorax by the side of the stomach and 

 oesophagus, slightly covering them on the right 

 side. Arrived at the superior part of the 

 abdomen, it is again bent, passes by the side 

 of the oesophagus, ascends rather behind the 

 branchial sac, and terminates at the inferior 

 part of the cloaca (w), at about the level of the 

 antepenultimate row of the branchial stigmata. 

 Throughout its length the intestine preserves 

 nearly an equal calibre, but varies in its ap- 

 pearance, and is divisible into three portions. 



The first part, the duodenum, succeeding to 

 the stomach and forming the intestinal loop, 

 is colourless and transparent. The next por- 

 tion, placed on a level with the stomach, but 

 on the ventral side of the body, is on the 

 contrary of a dull-yellow colour, and the 

 tissue of its walls has a glandular appearance. 

 This M. Milne-Edwards considers to be the 

 hepatic portion of the alimentary canal, and 

 comparable to the organ known in insects as 



the chylific ventricle. Lastly, in its third 

 portion, the intestine again becomes mem- 

 branous and colourless : here the fsecal matter 

 is collected into brownish pellets, and from its 

 functions and position this portion of the tube 

 is termed the great intestine, or rectum. 



To the right of the intestinal loop is placed a 

 glandular mass, the chief part of which is formed 

 by the ovary, recognised by its vesicles, and by 

 the eggs of different degrees of development 

 contained in it. Below the ovary is the 

 testicle, a mass of whitish and ramifying fila- 

 ments, spread out in some degree on the 

 intestine. From this glandular mass arises a 

 milk-white filiform canal, which ascends be- 

 tween the stomach and the intestine, passes 

 on the left side of the oesophagus, and opens 

 into the cloaca, near the orifice of the rectum. 

 This canal contains a silvery white fluid, 

 crowded with spermatozoa, and must be con- 

 sidered as a vas deferens. There being no other 

 visible duct leading from the ovary, this canal 

 perhaps serves also as an oviduct. The eggs are 

 minute, circular, and of agreenish yellow colour. 



The heart is situated at the inferior part of 

 the abdomen, lying against the intestine and 

 the ovary, to the right and in front of the 

 former, at the posterior part of the cavity 

 of the mantle, between the tunic and the 

 intestine, and much resembles that of Ascidia 

 intestinalis. It is enclosed in a membranous 

 sac or pericardium, and is cylindrical in form ; 

 its superior extremity is on a level with the 

 centre of the stomach, and its lower ex- 

 tremity, turned up a little behind, passes some- 

 times a little beyond the intestinal loop. 



The Perophora, first described by M. 

 Lister in Phil. Transact. 1834, p. 378., as a 

 " small Ascidia," is parasitic on Confervas, &c., 

 and appears to the eye like minute lumps of 

 pellucid jelly, with a spot of orange and grey. 

 The group consists of several individuals, 

 each having its own circulatory, respiratory, 

 and digestive systems, but fixed on a pedicle 

 that branches from a common creeping stem, 

 and all are connected by a circulation that 

 extends throughout. They are very trans- 

 parent, and their interior is easily seen. The 

 two orifices are very short tubes ; the bran- 

 chial is at the summit, and the anal is a little 

 lower down. The longest diameter from the 

 peduncle to the space between the openings is 

 about '085 inch (see Jig. 306. A and B, p. 623. 

 Vol. I. CILICI). The test is subcartilaginous 

 and tough, more pliable near the orifices. It is 

 lined internally with the soft mantle, in which 

 a ramifying circulation is very distinct. A 

 great part of the interior is occupied by the 

 branchial sac, which is subcylindrical, flattened 

 at the sides, and has its axis vertical. It is 

 united to the mantle above and behind ; the 

 juncture, beginning in front of the oral open- 

 ing, extends backwards on each side of it, 

 and then downwards in two lines ; between 

 these, along the middle of the back, is a 

 vertical compound stripe. At the bottom, the 

 branchial sac appears to be in contact with 

 the mantle, but at its sides and front a vacant 

 space is left between them, communicating 



