152 HISTORY OF THE HUMAN BODY 



The visceral skeleton is associated with the anterior portion 

 of the alimentary canal and the parts derived from it, and 

 seems to have developed primarily along the sides of the 

 pharynx for the regulation of those most primitive of verte- 

 brate respiratory organs, the gill-slits. Although Amphloxus 

 possesses a very regular and quite complicated system of 

 skeletal bars to support its eighty or ninety pairs of gill-slits, 

 these cannot as yet be brought into definite relationship with 

 the visceral skeleton of the higher vertebrates ; the same may 

 be said of the visceral skeleton of the cyclostomes, which is in 

 the form of a complicated pharyngeal basket, bearing little 

 apparent resemblance either to the skeletal bars of Amphioxus 

 or to the succession of simple arches characteristic of the 

 fishes. It is in the selachians that we first meet with a vis- 

 ceral skeleton to which that of the higher. forms can be cer- 

 tainly referred, and it is here, therefore, that the morphologi- 

 cal history of the vertebrate visceral skeleton must start. 



It consists of a series of pairs of cartilages, more or less 

 modified from the form of simple rods, and alternating with 

 gill-slits that open from the exterior into the pharyngeal 

 cavity. In most selachians there are seven well-developed 

 pairs of these, besides a few cartilages that may represent 

 rudiments of others, but as in two very primitive genera 

 there are, respectively, eight and nine regular pairs, besides the 

 rudiments, eleven or twelve original pairs can be accounted 

 for, thus suggesting a former condition with a large number 

 of gill-slits, a supposition that compares well with the testi- 

 mony furnished by Amphioxus. The selachian condition, 

 showing all the pieces that may be accredited to the visceral 

 skeleton, together with the chondrocranium, is given in Fig. 

 40, somewhat diagrammatized from an actual preparation. 

 It may be safely assumed that at one time these visceral arches 

 were all similar to one another, associated with similar gill- 

 slits and all gill-bearing, although important modifications 

 have now taken place in certain ones of them. These modifica- 

 tions are of the highest importance in this history and may 

 now be considered, with constant reference to the figure. 



