TUNICATA 



187 



-k-s 



end 



Bran- 

 chial sac 



neigh- 



matrix, containing cells of various shapes ; it is frequently 



pigmented and is penetrated by numerous lacunae, in which 



the blood flows. In- 



side the mantle, in 



all parts of the body, 



except along the ven- 



tral edge, there is a 



cavity, the atrial or 



peribranchial cavity, 



which opens to the 



exterior by the atrial 



aperture. This cavity 



is lined by a layer of 



cells derived origin- 



ally from the ecto- 



derm 1 and directly 



continuous with that 



layer through the vd- 



atrial aperture (fig. 



5) ; consequently the 



mantle is covered both & a _____ 



externally and inter- 



nally by ectodermal 



cells. 



The branchial aper- 

 ture ( mout h) leads in- 

 * * e branchial si- 



phon (buccal Cavity FIG. 4. Diagrammatic dissection of A. aunlvla to 



Or Stomodaeum), and bra^chia? aperture"'; a, anus; b%, bran'chill 

 sac ; dl, dorsal lamina ; dt, dorsal tubercle ; 

 end, endostyle; *, heart; i, intestine; ra, 

 mantle ; nj, nerve ganglion ; or, oesophagus ; 

 aa, oesophageal aperture ; op, ovary ; pbr, 

 peribranchial cavity ; r, rectum ; rf, stomach ; 

 t, test ; (n, tentacles ; rd, vas deferens ; gl, 

 subneural gland. (Original.) 



this opens into the 



anterior end of a very 



large cavity (the bran- 



chial sac) which ex- 



tends nearly to the 



posterior end of the body (see figs. 4 and 5). This branchial 



sac is an enlarged and modified pharynx, and is therefore 



properly a part of the ali- 



mentary canal. The oeso- 



phagus opens from it far 



back on the dorsal edge (see 



below, p. 6 1 2). The wall of 



the branchial sac is pierced 



by a large number of ver- 



tical slits, the stigmata, 



placed in numerous trans- 



verse rows. These slits 



place the branchial sac in 



communication with the 



end 



FIG. 5. Diagrammatic longitudinal (A) and transverse (B) sections through 

 Axulia to show the position of the ectoderm and the relations of the bran- 

 chial and peribranchial cavities. The lettering is the same as for fig. 4. B 

 represents a section taken along the dotted line A-B in A. (Original.) 



peribranchial or atrial cavity, which lies outside it (fig. 5, 

 B). Between the stigmata the wall of the branchial sac 

 is traversed by blood-vessels, which are arranged in three 

 regular series (fig. 6), (1) the transverse vessels, which 

 run horizontally round the wall and open at their dorsal 

 and ventral ends into large longitudinal vessels, the dorsal 

 and ventral sinuses ; (2) the fine longitudinal vessels, which 

 run vertically between adjacent transverse vessels and open 

 into them, and which bound the stigmata ; and (3) the 

 internal longitudinal bars, which run vertically in a plane 



1 According to E. van Beneden and Julin's recent investigations (jo) 

 only the outer wall of the atrium is lined with epiblast, the inner wall 

 being derived from the hypoblast of the primitive branchial sac. 



internal to that of the transverse and fine longitudinal 



vessels. These bars communicate with the transverse 



vessels by short side J" !. 



branches where they 



cross, and at these 



points are prolonged 



into the lumen of the 



sac in the form of 



hollow papillae. The 



edges of the stigmata 



are richly set with 



cilia, which drive the 



water from the bran- 



chial sac into the 



peribranchial ca- 



vity, and so cause 



the currents that 



flow in through 



the branchial 



aperture and 



out through the 



atrial 



Fio. 6. A. Part of branchial sac of Ascidia from inside. 

 Along US Vent- B. Transverse section of same, tr, transverse vessel ; 

 <** connecting duct; *, horizontal membrane; il, 

 internal longitudinal bar ; Ir, fine longitudinal vessels; 



p, ]f, papillae ; tg, stigmata. A a 

 different stales. (From Herdmau, 



A and B are drawn to 

 Challenger Report.) 



ral f>rlcrf> trip wall 

 M t> a l 



of the branchial 

 sac is continn 



ous externally with the mantle (fig. 5, B), while internally 

 it is thickened to form two parallel longitudinal folds 

 bounding a groove, the " endostyle," hypobrancbial groove, Endo- 

 or ventral furrow (figs. 4, 5, end). The endoderm cells style. 

 which line the endostyle are greatly enlarged at the 

 bottom and on parts of the sides of the furrow so as to 

 form projecting pads, which bear very long cilia. It is 

 generally supposed that this organ is a gland for the pro- 

 duction of the mucous secretion which is spread round the 

 edges of the branchial sac and catches the food particles in 

 the passing current of water; but it has recently been 

 pointed out that there are comparatively few gland cells in 

 the epithelium of the endostyle, and that it is more prob- 

 able that this furrow is merely a ciliated path along which 

 the mucous secretion (produced possibly by the subneural 

 gland) is conveyed posteriorly along the ventral edge of 

 the branchial sac. At its anterior end the edges of the Peri- 

 endostyle become continuous with the right and left halves pharyn- 

 of the posterior of two circular ciliated ridges, the peri- 

 pharyngeal bands, which run parallel to one another 

 round the front of the branchial sac. The dorsal ends of 

 the posterior peripharyngeal band bend posteriorly (en- Dorsal 

 closing the epibranchial groove), and then join to form lamina. 

 the anterior end of a fold which runs along the dorsal edge 

 of the branchial sac as far as the oesophageal aperture. 

 This fold is the dorsal lamina (figs. 4, 5, dt). It probably 

 serves to direct the stream of food particles entangled in 

 a string of mucus from the anterior part of the dorsal 

 lamina to the oesophagus. In many Ascidians this organ, Dorsal 

 instead of being a continuous membranous fold as in J.languet 

 mentula, is represented by a series of elongated triangular 

 processes the dorsal languets, one attached in the dorsal 

 median line opposite to each transverse vessel of the 

 branchial sac. The anterior peripharyngeal band is a 

 complete circular ridge, having no connexion with either 

 the endostyle or the dorsal lamina. In front of it lies the 

 prebranchial zone, which separates the branchial sac behind 

 from the branchial siphon in front. The prebranchial 

 zone is bounded anteriorly by a muscular band the pos- 

 terior edge of the sphincter muscle, which bears a circle 

 of long delicate processes, the tentacles (figs. 4, 7, 8, tn). Ten- 

 These project inwards at right angles so as to form a net- tades. 

 work across the entrance to the branchial sac. Each 

 tentacle consists of connective tissue covered with epithe- 



