366 E. W. MACBRIDE. 



is giving rise to the subatrial muscle. The ectodennic thickening is hollowed 

 out to form a lymph-space. 



Fig. 8. — Transverse section through the posterior part of the pharyngeal 

 region of the same larva as that from wliich Pig. 4 was taken. Magnification 

 1100 diameters. 



Tig. 9. — Transverse section through the posterior pharyngeal region of a 

 larva, with numerous gill-slits, older than that figured in Eig. 5, but younger 

 than the larva represented in Fig. 7. The atrial ridges are well marked. 

 Magnification 350 diameters. 



Fig. 10. — A section from the same series as that figured in Fig. 7, through 

 the posterior pharyngeal region of the larva. The atrial cavity is formed. 

 The metapleural coelom has become solid, and in each atrial ridge there is a 

 lymph-space. 



