580 



COMPARATIVE ANATOMY 



CHAP. 



chondroid tissue a certain similarity to cartilage. This chondroid 

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



B'iG. 4i;2.— Gill silts and brancMal skeleton of an Enteropneustan. The six liindeniiost 

 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. 



m 



the 



genera 



boscidal skeleton, which always remains at its centre, 



SchizocarcUum and Glandicsps. 



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 

 coelom 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 

 of the septal edge, which is turned 

 towards the cavity of the branchial in- 

 testine. This septal prong forks at 

 its free lower end, 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 



Vv,. 463.— The three anterior forks ot 

 the branchial skeleton in Balanoglossus 

 Kowalevskii (after Spengel). The most 

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

 terior lingual prong ; 2, a septal prong (in 

 its origin double) ; 3, an anterior lingual 

 prong. 



