436 



INTRODUCTION TO CRANIATA 



[CH. 



of the lymph-system from the blood-system is that the haemoglobin 

 is not diffused in the fluid of the blood, but is carried in cells 

 which have no power of movement in themselves. Did these cells 

 enter the lymph-system they would speedily block its finer channels. 

 The supply of amoebocytes to both blood and lymph is provided 

 for by widely distributed actively growing nodules of cells which 

 bud off amoebocytes into the adjacent lymph-channels. These 

 packets of cells are called lymphatic glands : the largest collection 



FIG. 215. Dorsal view of head of Scyllium caniculaxl. The right orbit has 

 been exposed so as to show the muscles that move the eye and the second 

 and fourth nerves. 



1. Lens of the eye. 2. Superior rectus muscle of the eyeball. 3. Ex- 

 ternal (or posterior) rectus muscle. 4. Inferior rectus muscle. 5. In- 

 ternal (or anterior) rectus muscle. 6. Inferior oblique muscle. 

 7. Superior oblique muscle ; the slender nerve entering this muscle is the 

 fourth cranial. 8. Second cranial or optic nerve, the nerve of sight. 



is in the spleen, an organ having several other functions, which is 

 attached to the mesentery just dorsal to the posterior end of the 

 stomach. 



The muscles of the Craniate like those of 'the Cephalochorda 

 are developed from the inner walls of a series of dorsal 



Coelom. 



coelomic pockets, in a word, from myotomes. Unlike 

 that of the Cephalochorda the trunk coelom does not become at 

 first completely divided into separate sacs, the ventral portions 

 of which fuse later. In the Craniate this stage is skipped in 



