516 THE ANATOMY OF INVERTEBRATED ANIMALS. 



ried through the oral opening, at right angles to a transverse 

 plane passing through its centre, the mouth is found to open 

 into a large pharyngeal dilatation, termed the hranchial sac 

 (Fig. 148, d). A series of simple or pinnatifid tentacles (Fig. 

 148, c) is seen encircling the oral aperture at some little dis- 

 tance within the margin of the lip, which is usually divided, 

 like that of tlie atrial opening, into four or six lobes. Imme- 

 diately behind the tentacular circlet is a ciliated pharyngeal 

 band. 



On that side of the branchial cavity which is farthest away 

 from the atrial opening, a pair of delicate lip-like folds ex- 

 tend, parallel with one another, from the peripharyngeal band 

 alono- the middla line of the branchial sac as far as the open- 

 ing of the oesophagus at the opposite end of the branchial 

 sac. The interspace between these leads into a fold of the 

 endoderm, lined by a thick epithelium and forming the endo- 

 style, and, in the middle line of \X\q peripharyngeal hand, on 

 the same side as the atrial aperture, there is a tubercular ele- 

 vation, which contains a ciliated cavity, and answers to the 

 ciliated sac of Appendicularia. The walls of this sac are va- 

 riously folded, and, consequently, the surface of the tubercle 

 presents a more or less complicated pattern. Continued back- 

 ward in the middle line as far as the oesophageal aperture on 

 this side of the branchial sac, there are sometimes one, some- 

 times two, longitudinal lamellae — the hypopharyngeal folds ; 

 or there may be merely a ridge surmounted by a series of ten- 

 tacles, termed lancjuets (Fig. 148, e). The languet which is 

 nearest the ciliated sac is often the largest of the series. Be- 

 hind the peripharyngeal band, the lateral walls of the pharyn- 

 geal, or branchial, sac are perforated by small elongated ap- 

 ertures — the stigmata — the edges of which are fringed with 

 long cilia ; and, by means of these apertures, the cavity of 

 the sac communicates with the atrium. 



The stigmata are arranged in transverse rows, and are 

 usually very numerous. The reticulated wall of the branchial 

 sac may be strengthened by longitudinal lamellae, or it may 

 be raised into few and distant, or many and close-set, folds. 

 In some cases pajdllre of a complicated form are developed 

 from the inner surface of the sac, and its outer wall is always 

 connected by vascular trabeculae with the parietal wall of the 

 atrium. In some cases {]\Iolgvla), the stigmata, instead of 

 being elongated meshes, are coiled spirally. The atrial cham- 

 ber (Fig. 148, k), into which the branchial stigmata open, is 

 shown by laying it open from the atrial aperture, in the same 



