SKELETON OF UROCHORDATA AND CEPHALOCHORDATA. 51 ' 



the notochord forms a small diverticulum growing forwards 

 from the alimentary canal into the proboscis stalk. 



Recent researches on Phoronis 1 show the existence in the 

 collar region of the larva (ActinotrocJia) of a paired organ, 

 which is regarded by its discoverer as representing a double 

 notochord. 



SUBPHYLUM B. UROCHORDATA (TUNICATA). 



Skeletal structures of epiblastic and hypoblastic origin 

 occur in the Urochordata. Most Tunicates are invested by 

 a thick gelatinous test which often contains calcareous spicules, 

 and serves as a supporting organ for the soft body. The cells 

 of this test are mesodermal in origin. 



In larval Tunicata and in adults of the group Larvacea 

 the tail is supported by a typical notochord, which is confined 

 to the tail. In all Tunicata except Larvacea all trace of 

 the notochord is lost in the adult. 



SUBPHYLUM C. CEPHALOCHORDATA. 



This subphylum includes the well-known genus Amphioxus 2 . 



i 



5 8 



FIG. 3. DIAGRAM OF THE SKELETON OF Amphioxus lanceolatus x 3 

 (after a drawing in the Index collection at the Brit. Mus.). 



1. skeleton of dorsal fin. 5. branchial skeleton. 



2. notochord. 6. septa separating the myo 



3. neural tube. tomes. 



4. buccal skeleton. 7. skeleton of ventral fin. 



In Amphioxus the skeleton is very simple. It contains no 

 trace of cartilage or bone and remains throughout life in a 



1 A. T. Masterman, P. R. Soc. Edinb. 189596, p. 59 ; and Anat. Anz. 

 1896, p. 266. 



2 See E. Ray Lankester, Quart. J. Micr. Sci. vol. xxix. n. s. 1889, p. 365. 

 W. B. Benham, Ibid. vol. xxxv. n. s. 1893, p. 97. J. W. Kirkaldy, Ibid, 

 vol. xxxvn. n. s. 1895, p. 303. The last-named writer divides the genus 

 into three subgenera. 



42 



