TUNICA T A 



187 



matrix, containing cells S various shapes ; it is frequently 

 pigmcntcd anil is penetrated by numerous lacuna 1 , 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 

 peribrancliial 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 l and directly 

 continuous with that 

 layer through the vd 

 atrial aperture (fig. 

 5) ; consequently the 



mantle is covered both g, a ' 



externally and inter- 

 nally by ectodermal 

 cells. <*-- 



Bran- The branchial aper- p lr 



chial sac ture ( m outh) leads in- 



iei<-h ^ branchial si- ^ r 



bourine phon (buccal Cavity Fio. 4. Diagrammatic dissection of A. mentula to 



i \ j show the anatomy, at, atrial aiierture ; br, 



organs, or StoniOdasum), and branchial aperture; n, anus; Irs, branchial 



this Opens into the sac <*'> dorsal lamina ; dt, dorsal tubercle 



j ' end, endostyle ; h, lieart ; i, intestine ; in 



anterior end Ot a very mantle ; ng, nerve ganglion ; a-, (Esophagus 



laro-p r-nvirWtlip ln-an a ' a > cesophageal aperture; OP, ovary; plr 



large Cavity (IK peribrancliial cavity ; r, rectum ; st, st.,,,,a,-l, 



chial Sac) which ex- t, test ; /, tentacles ; vd, vas deferens ; ngl, 



tends nearly to the subneural slani1 ' (Urisil 

 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. 612). 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 



d I--... 



:i . end 



B 



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

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

 chial and peribranchial cavities. The lettering is the same as fur fig. 4. li 

 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 dor>al 

 and ventral ends into large longitudinal vessels, the dorsal 

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

 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 hyiioblast of the primitive branchial sac. 



Trough 



Fin. Ci.A. Part of branchial sac of Aseidia from inside. 

 Jl. Transverse section of same, tr, transverse vessel ; 

 cd, connecting duct ; Tim, horizontal membrane ; il, 

 internal longitudinal bar ; Ic, fine longitudinal vessels; 

 p, p', papillfe ; sg, stigmata. A and B are drawn to 

 different scales. (From Herdmau, Challenger llcport.) 



internal to that of the transverse and fine longitudinal 

 vessels. Th<-se liars communicate with the transverse 

 vessels by short side 

 branches where they 

 cross, and at these. /. 

 points are prolonged 

 into the lumen of the 

 sac in the form of 

 hollow papilla?. The 

 edges of the stigmata n 

 are richly set with 

 cilia, which drive the 

 water from the bran- 

 chial sac into the 

 peribranchial ca- h ,"_ 

 vity, and so cause 

 the currents that 

 flow in through 

 the branchial 

 aperture and 

 out through the 

 atrial. 



Along its vent- 

 ral edge the wall 

 of the branchial 

 sac is continu- 

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

 it is thickened to form two parallel longitudinal folds 

 bounding a groove, the " endostyle," Lypobranchial 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 lias 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-p , 

 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 cesophageal aperture. 

 This fold is the dorsal lamina (figs. 4, 5, dl). 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 A. languets. 

 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, in). Ten- 

 These project inwards at right angles so as to form a net- tac ' es - 

 work across the entrance to the branchial sac. Each 

 tentacle consists of connective tissue covered with epithe- 



