48 ZOOLOGY SHCT. 



The mouth leads into the largest section of the enteric canal, 

 the pharynx (ph.), a high, compressed chamber extending through 

 the anterior half of the body. Its walls are perforated" 1 by more 

 than a hundred pairs of narrow oblique clefts, the gill- slits or 

 branchial apertures (br. cl.), which place the cavity of the pharynx 

 in communication with the atrium (see below). From the pos- 

 terior end of the pharynx goes off the tubular intestine (int.) which 

 extends backwards almost in a straight line to the anus. 



On the ventral wall of the pharynx is a longitudinal groove, the 

 endostyle (Fig. 742. A, .), lined by ciliated epithelium containing 

 groups of gland-cells. Like the homologous organ in Ascidia 

 (p. 17), the glands secrete a cord of mucus in which food parti- 

 cles are entangled and carried by the action of the cilia to the 

 intestine. A somewhat similar structure, the epipharyngcal groove, 

 extends along the dorsal aspect of the pharynx : its sides are 

 formed by ciliated cells, which, at the anterior end of the groove, 

 curve downwards, as the peripharyngeal bands, and join the 

 anterior end of the endostyle. 



From the ventral region of the anterior end of the intestine is 

 given off a blind pouch, the liver (Ir.) or hepatic caecum, which 

 extends forwards to the right of the pharynx : it is lined with 

 glandular epithelium and secretes a digestive fluid. 



The gill-slits (br. cl.) are long narrow clefts, nearly vertical in 

 the expanded condition, but very oblique in preserved and con- 

 tracted specimens hence the fact that a large number of clefts 

 always appear in a single transverse section (Fig. 742, A, Jed.). 

 The clefts are more numerous than the myomeres in the adult, 

 but correspond in number with them in the larva : hence they are 

 fundamentally metameric, but undergo an increase in number as 

 growth proceeds. 



The branchial lamellae (Fig. 743, br. sep., Fig. 742, A, kb.), or por- 

 tions of the pharyngeal wall separating the clefts from one another, 

 are covered by an epithelium which is for the most part endo- 

 dermal in origin, and is composed of greatly elongated and ciliated 

 cells. On the outer face of each lamella, however, the cells are 

 shorter and not ciliated, and are, as a matter of fact, portions of 

 the epithelial lining of the atrium, and of ectodermal origin. 

 Each lamella is supported towards its outer edge by one of the 

 branchial rods (Fig. 743, br. r.) already referred to. Those are 

 narrow bars united with one another dorsally by loops, but ending 

 below in free extremities which are alternately simple and forked. 

 The forked bars are the primary (br. r. 1), those with simple ends 

 the secondary (br. r. %) branchial rods, and the lamellae in which they 

 are contained are similarly to be distinguished as primary lamellae 

 (br. sep. 1) and secondary or tongue-lamella (br. sep. 2). In the young 

 condition the two clefts between any two primary lamellae are 

 represented by a single aperture : as development proceeds a down- 



