326 MANUAL OF ZOOLOGY sect. 



body. Its walls are perforated by more than a hundred pairs of narrow 

 oblique clefts, the gill-slits or branchial apertures (br. c), which place 

 the cavity of the pharynx in communication with the atrium (see 

 below). From the posterior end of the pharynx goes off the tubular 

 intestine (ini) which extends backwards, almost in a straight line to 

 the anus. 



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

 endostyle, lined by ciliated epithelium containing groups of gland-cells. 

 Like the homologous organ in Ascidia (p. 317), the glands secrete a 

 cord of mucus in which food particles are entangled and carried by the 

 action of the cilia to the intestine. A somewhat similar structure, the 

 epipharyngeal 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 peri-pharyngeal 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 for- 

 wards 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 contracted 

 specimens — hence the fact that a large number of clefts always appear 

 in a single transverse section (Fig. 205). 



The branchial septa or lamella? (Fig. 204, br. sep), or portions of the 

 pharyngeal wall separating the clefts from one another, are covered 

 by an epithelium composed, except on the outer face, of greatly elon- 

 gated and ciliated cells. Each septum is supported towards its outer 

 edge by one of the chitinoid branchial rods (br. r) already referred 

 to. 



The gill-clefts lead into a wide chamber occupying most of the space 

 between the body-wall and the pharynx and called the atrium (Fig. 

 204, atr). It is crescentic in section, surrounding the ventral and 

 lateral regions of the pharynx, but not its dorsal portion. It ends 

 blindly in front; opens externally, behind the level of the pharynx, by 

 the atriopore (atrp) ; and is continued backwards by a blind, pouch- 

 like extension (atr) lying to the right of the intestine. As in Ascidia 

 the cilia lining the gill-clefts produce a current setting in at the mouth, 

 entering the pharynx, passing thence by the gill-slits into the atrium 

 and out at the atriopore. The current, as in Tunicata and Balano- 



