446 



Causes and Course of Organic Evolution 



IcP, end blindly against or in the mesoderm substance. In the 

 Cyclostomata the depressions are continuous by their inner 

 faces ^\^th the pharyngeal cavity, while the branchial lamellae 

 are represented by a series of vascular horizontal and parallel 

 ridges, radiating outwards along the roof, floor, and lateral 

 walls of each gill sac, and invested by an epithelium that is 

 partially ciliated. In lower Amphibia the "gill pouches" or 

 depressions are also continuous from without into the pharynx 

 (Fig. 19e). In those genera that develop external gills first, 

 these arise as dorsal lamellae, which instead of merely projecting 



a 



b 



— '"^ -J 



ep 



Fig. 19. — a, b, section of ciliated head-grooves in Turbellaria rhabdocoela; 

 c, section of ciliated furrow of Nemertean; d, section of branchial sac of Cyclo- 

 storae; e, do. of caecilian Amphibian. 



into the pouch, as in nemerteans and cyclostomes, push out- 

 ward and may attain great length as in Ichthyophis of the 

 Apoda or Cseciliada so carefully studied by Sarrasin. The 

 internal gills are lamellae that arise on the inner ventral sur- 

 face of each pouch, later as a rule than do the external, and 

 which become greatly more branched and complex than in 

 the two more ])rimitive grou]>s of the cyclostomes and the 

 nemerteans. 



