CHAPTER III. 



SKELETON OF HEMICHORDATA, UROCHORDATA, 

 AND CEPHALOCHORDATA. 



SXJBPHYLUM A. HEMICHORDATA. 



THE subphylum includes three genera, Balanoglossus 1 , Ce- 

 phalodiscus and Rhabdopleura ; and perhaps a fourth, Phoronis. 



The skeletal structures found in Balanoglossus 2 are all 

 endoskeletal. They include 



(1) The notochord. This arises as a diverticuluni from 

 the alimentary canal which grows forwards into the proboscis 

 and extends beyond the front end of the central nervous 

 system. It is hypoblastic in origin and arises in the same way 

 as does the notochord of Ampkioxus. Its cells become highly 

 vacuolated and take on the typical notochordal structure 3 . The 

 cavity of the primitive diverticulum becomes obliterated in front, 

 but behind it opens throughout life into the alimentary canal. 



(2) The axial skeletal rods. These are a pair of chi- 

 tinous rods which lie ventral to the notochord and in the collar 

 region unite to form a single mass. 



(3) The branchial skeleton. The gill bars separating 

 the gill slits from one another are strengthened by chitinous 

 rods in a way closely similar to that in Ampliioxus, But 

 between one primary forked rod and the next there are two 

 secondary unforked rods not one, as in Amphioxus. 



(4) The chondroid tissue. This is of mesoblastic origin 

 and may be regarded as an imperfect sheath for the notochord. 



In Cephalodiscus and Rhabdopleura as in Balanoglossus 



1 The name Balanoglossus is used here in its widest sense to include 

 all the Enteropneusta. 



2 See W. Bateson, Quart. J. Micr. Sci. n. s. vol. xxiv. 1884, p. 208 and 

 later; also E. W. Macbride, Ibid. vol. xxxvi. 1894, p. 385. 



3 See p. 52. 



