580 



COMPARATIVE ANATOMY 



CHAP. 



chondroid tissue a certain similarity to cartilage. This chondroid 

 tissue is most developed, forming a mass thicker than the pro- 



S-Ji 



FIG. 462. Gill slits and branchial skeleton of an Enteropneustan. The six hindermost 

 gills seen from the intestine, the three posterior in the act of forming, diagrammatic. The black 

 parts represent the U-shaped gill slits ; the dotted parts, the skeletal forks. 1, Branchial tongue ; 

 2, branchial septum ; 3, anterior prong ; 4, median or septal prong ; 5, posterior lingual prong of 

 a three-pronged skeletal fork. 



boscidal skeleton, which always remains at its centre, in the genera 



Schizocardium and Glandiceps. 



B. The Branchial skeleton (Figs. 462 and 463). (Cf. here pp. 



567 and 568 on the gill slits, the branchial septa, and the branchial 



tongues.) 



The branchial skeleton here, again, consists of local thickenings of 



the limiting membrane, which separates 

 the epithelium of the branchial intes- 

 tine from the visceral wall of the trunk 

 coalom of the branchio-genital region. 

 These thickenings are in the form of 

 upright three -pronged skeletal forks, 

 which are arranged on each side, in a 

 single longitudinal row, throughout the 

 whole length of the branchial region. 

 The number of forks corresponds with 

 that of the gills. The free ends of the 

 prong are turned downwards, and the 

 connecting piece upwards. The three 

 prongs of a fork are arranged as 

 follows. The middle prong lies in a 

 branchial septum, under the surface 



FIG. 463. The three anterior forks of O f the septal edge, which is turned 



l ^ ^ i t y <* the branchial in- 



tCStme. This Septal prong f Orks at 



Jtg f ree lower Clld, giving off a short 

 . , 



anterior and a posterior branch. 



The anterior prong of a fork lies 

 on the posterior wall of the branchial tongue, immediately in front of 

 the septum ; the posterior prong in the anterior wall of the branchial 



anterior (I) has only two prongs. 1, A pos- 

 terior lingual prong ; 2, a septal prong (in 

 its origin double); 3, an anterior lingual 



p ron g 



