PHYLUM CHORDATA 



325 



these cells is a notochordal 

 sheath of connective tissue 

 which is produced dorsally 

 into a canal for the ner- 

 vous system. 



The oral hood is sup- 

 ported by a ring (Fig. 204, 

 sk) of cartilaginous con- 

 sistency, made up of sepa- 

 rate rod-like pieces ar- 

 ranged end to end, and 

 corresponding in number 

 with the cirri. 



The pharynx is sup- 

 ported by delicate oblique 

 rods of a chitinoid mate- 

 rial, the gill-rods (dr. r). 

 The dorsal fin is supported 

 by a single series, and the 

 ventral fin by a double 

 series, of fin rays (dors, 

 f. r), short rods of con- 

 nective tissue. 



The mouth (mtK), as 

 already mentioned, lies at 

 the bottom of the vestibule 

 or cavity of the oral hood 

 {or. Ad). It is a small cir- 

 cular aperture surrounded 

 by a membrane, the velum ^ ? 

 (vl) acting as a sphincter, g 

 the free edge of which is "8 

 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 





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