324 MANUAL OF ZOOLOGY SECT. 



these cells is a notochordal sheath of connective tissue which is produced 

 dorsally into a canal for the nervous system. 



The oral hood is supported by a ring (Fig. 191, sk. ) of cartilaginous 

 consistency, made up of separate rod-like pieces arranged end to end, 

 and corresponding in number with the cirri. 



The pharynx is supported by delicate oblique rods of a chitinoid 

 material, the gill-rods (br. r.). The dorsal fin is supported by a single 

 series, and the ventral fin by a double series, of fin-rays (dors. f. r., 

 vent. f. r.) short rods of connective tissue. 



The mouth (mth.) as already mentioned, lies at the bottom of the 

 vestibule or cavity of the oral hood (or. hd.). It is a small circular 

 aperture surrounded by a membrane, the velum (vl.) acting as a 

 sphincter, the free edge of which is produced into a number of velar 

 tentacles (vl. /.). 



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 by more than a 

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

 apertures (br. <:/.), which place the cavity of the pharynx in communica- 

 tion with the atrium (see below). From the posterior 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, lined by ciliated epithelium containing groups of gland-cells. 

 Like the homologous organ in Ascidia (p. 316), 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 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 or hepatic cacum (lr.), 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. 



The branchial septa or lamellae (Fig. 191, br. sep.) ; i.e. the portions of 



