618 FRANCIS M. BALDWIN 



body in 17 mm. Siredon larva described by Maximow (136 x 87 

 micra). The other two, however, are a httle smaller in Ambly- 

 stoma than those of Siredon. 



The third thymus body (fig. 26) begins 140 micra caudad to the 

 posterior wall of the ear, where it lies in the connective tissue in 

 the region above the pharynx, a little medial and anterior to the 

 posterior caudal tip of the cartilage of the first branchial arch. 

 The first efferent branchial artery emerges from the medial sur- 

 face of the first branchial cartilage near its caudal tip, and passes 

 forward and medially, coursing just below the third thymus body, 

 and, after giving off the internal carotid artery, turns medially to 

 become the radix aortae. A little ventral to the anterior end of 

 the thymus body — between it and the blood vessel just described 

 — the glossopharyngeal nerve {IX, fig. 41) passes to the lateral 

 surface of the caudal tip of the first branchial cartilage, where it 

 divides into its pre- and post-trematic rami. In a similar manner, 

 a ramus (the first branchial nerve of the vagus, ni,) passes 

 laterally, caudad and a little ventrally to the posterior end of the 

 third thymus body, and breaks up into its pre- and post-trematic 

 branches. 



The fourth thymus body (fig. 27) bears the same relation to the 

 caudal tip of the cartilage of the second branchial arch, as the 

 third thymus body does to the first, but, since the arches turn 

 upwards and inwards at their caudal extremities, this thymus 

 body is a little higher as well as a little more medial in position. 

 The second efferent branchial artery passes from the inedial sur- 

 face of the caudal tip of the second arch, just back of and a little 

 below the fourth thymus body, and courses medially to join the 

 radix some distance in front. The second branchial nerve (rio) of 

 the vagus, passes into the gill region just back of the caudal end 

 of this body. 



The fifth thymus body (fig. 28) is somewhat dorsal and caudal 

 to the junction of the third and fourth branchial cartilages at their 

 most caudal ends, and, since these are directed medially as well as 

 dorsally (see also fig. 41), this body has been forced into the space 

 between them and the muscles, especially the dorso-laryngeus 



