44 JZOOLOGV 



tissue known as notochordal tissue, formed of large vacuolated cells 

 extending from side to side of the notochord, and having the 

 nuclei confined to its dorsal and ventral regions. Around these 

 cells is a structureless layer, secreted by the cells, enclosed in a 

 notochordal sheath of connective-tissue, which is produced dorsally 

 into an investment for the canal enclosing the central nervous 

 system. The notochord, like the parenchyma of plants, owes its 

 resistant character to the vacuoles of its component cells being 

 tensely filled with fluid, a condition of turgescence being thus pro- 

 duced. 



The oral hood is supported by a ring (Fig. 752, sk.) of cartilaginous 

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

 end, and corresponding in number with the cirri. Each piece sends 

 an offshoot into the cirrus to which it is related, furnishing it with 

 a skeletal axis. 



The pharynx is supported by delicate oblique rods of a firm 

 material, apparently composed of agglutinated elastic fibres, the 

 gill-rods (br. r.). These will be most conveniently discussed in 

 connection with the pharynx itself. 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, continuous 

 with the investment of the neural canal and separated from one 

 another by small cavities (lymph-spaces). 



Digestive and Respiratory Organs. 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.), which acts as a sphincter, and has 

 its free edge produced into a number of velar tentacles (vl. t.}. 



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. cl.), 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 (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. 751, A, e.), 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-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 down- 

 wards, as the peripharyngeal bands, and join the anterior end of 

 the endostyle. 



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



