THE LOWER VERTEURATA. 



201 



Instance of a purely endoplastic tissue. Instead of being 

 soft as its remains are in the dogfish it is at least as firm as 

 cartilage, and even more flexible and elastic, so that it forms 

 an excellent basis for the myotomes to act upon in swim- 

 ming. Its anterior end extends right up to the tip of the 

 snout, and thus makes it easy for the animal to burrow in 

 the sand (2). A notochord occurs in the embryos of all 

 higher vertebrata, but it never extends in front of the mid- 

 brain, and it is usually replaced by vertebrae as develop- 

 ment goes on. 



The prse-oral hood and cirri are supported on a jointed 

 framework, composed of notochordal 

 tissue. Possibly the labial cartilages 

 of the dogfish represent this prse-oral 

 skeleton. 



The gill-arches are supported by a 

 series of bars, which are of quite 

 different chemical composition from 

 any other Vertebrate skeleton. Their 

 material is insoluble in hot caustic 

 potash solution, and must therefore 

 be very similar to the chitin which 

 forms the exoskeleton of insects and 

 many other Invertebrates. This 

 branchial skeleton consists of a series 

 of primary bars, lying in the primary (From wniey.) The dotted 

 arches, and forking below ; of tongue- ISKSLS! 3 "^^ ^ 

 bars, lying in the secondary arches, 



and not forking ; and of cross-bars lying in the connexions 

 which unite one primary arch to another and cut up the 

 slits (fig. 108). 



The median fin is supported by a series of gelatinous fill- 

 rays, more numerous than the myomeres. Ventrally, 

 between atriopore and anus these are double, indicating that 

 the fin represents two folds united in the middle line. The 

 metapleural folds have no fin-rays. 



Lastly the connective tissue which surrounds and separates 

 all the organs is very strong and plays the part of a skeleton 

 both in the way of protection and in that of affording a 

 firm basis for muscular contraction. 



Fig. 103. PART OF 



SKELETON OF PHARYNX OF 



AMPHIOXUS. 



