228 TOWARDS THE HUMAN FORM 



times take. These, too, however, result from the reactions 

 of a liquid on an organism in the act of swimming, reactions 

 which modify the branchial region in particular. From this 

 point of view three types of Fishes can be distinguished : 

 Marsipobranchs, Elasmobranchs, and Ctenobranchs. The 

 Marsipobranchs are represented to-day by three genera 

 only : the Bdellostomas, Myxines, and the Lampreys. The 

 Elasmobranchs include numerous genera belonging to the 

 Sharks, Rays, and Chimaeroids ; the Ctenobranchs comprise all 

 the rest. In the Marsipobranchs the branchial system consists 

 of two almost symmetrical series of sacs placed behind the head, 

 independent of one another, and each communicating directly 

 or indirectly, by means of afferent and efferent ducts, with 

 the oesophagus and the exterior respectively. A kind of 

 cartilaginous grille supports each sac. In the Elasmobranchs 

 these gill-sacs become flattened, unite with one another, 

 and become indistinguishable from the ducts in such a way 

 that each sac communicates with the oesophagus by one 

 slit and with the exterior by another. Cartilaginous arches, 

 from which numerous rays branch out, support the thick 

 partitions resulting from the union of the walls of the gill- 

 sacs. In the Ctenobranchs the partitions are reduced to the 

 supporting and now osseous arches, to the dependent rays, 

 and to the highly vascular tissues which clothe the arch and 

 the rays it supports, while leaving these rays completely 

 independent. The Bdellostomas have as many as fourteen 

 branchial sacs, and their embryos show indications of many 

 more ; the Lampreys have seven on each side and Myxine 

 has only six, opening to the outside by a single duct. Seven 

 branchial slits are observed also in Sharks of the genus 

 Heptanchus ; Hexanchns and Chalmydoselackus have but 

 six, while five is the number characteristic of all the others, 

 as well as of Rays and the Chimaeroids. Finally in the Cteno- 

 branchs there are generally four branchial arches, rarely less, 

 and the rudiments of a fifth. This simple enumeration will 

 suffice to show that the branchial region constantly diminishes 

 as the type of Fish becomes more advanced. 



We have pointed out (p. 130) that the cause of this shortening 

 was the resistance offered by the water to the progression 

 of the animal propelled forward by the movements of its 

 tail. As is always the case, some Fishes have resisted this 



