580 



COMPARATIVE ANATOMY 



chondroid tissue a certain similarity to cartilage. This chondroid 

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



Pig. 462.— Gill Slits and branchial skeleton of an Enteropneustan. The six hiiideriuost 

 K'ills seen from the intestine, the three posterior intlie act of fcirniing, iliagrammatic. Tlie black 

 parts represent tlie 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 



Schizocardinm and Glandkeps. 



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 

 crelom 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 

 Fici. 463.— The three anterior forks of of the septal edge, which is turned 

 the branchial skeleton in Baianogiossus towards the cavity of the branchial in- 



Kowalevskii (after Spengel). The most J 



anterior (1) has only two prongs. 1, A pos- tCStine. ThlS Septal proilg foi'ks at 



terior lingual prong ; 2, a septal prong (in jts free lower end, giving off a short 



its^origin double); 3, an anterior lingual j^^terior 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 



