THE BEANCHIAL BLOOD-VESSELS. 175 



arteriosus (I'ig. 80, af.4), really a brancli from that of the third 

 arch, does not commence to form until after the mouth of the 

 tadpole is established. 



e. T ]he vesse ls of the hyoid arch. In the hyoid arch, at an 

 early stage ot- development, vessels are present which agree 

 closely in relations and in arrangement with those of the 

 branchial arches ; but which, after developing up to a certain 

 point, undergo degenerative changes, and in the later stages of 

 tadpole existence lose all trace of their original disposition. 



In tadpoles of 5 mm. length, not long before hatching, the 

 hyoid vessels consist of: — (i) an elongated efferent lacunar vessel, 

 lying parallel to, and in front of, the efferent vessel of the first 

 branchial arch; and (ii) a very small diverticulum from the 

 dorsal aorta, which lies opposite the upper end of the lacunar 

 vessel, but does not quite meet this. 



In newly hatched tadpoles two further changes have oc- 

 curred : — (i) a small blind diverticulum arises from the truncus 

 arteriosus, just in front of the diverticulum for the first 

 branchial arch ; and (ii) the efferent lacunar vessel has become 

 obliterated about the middle of its length, and so divided into 

 two separate portions, dorsal and ventral. Of these, the dorsal 

 one has no communication with any other vessel, although it 

 lies very close to' the diverticulum from the aorta ; while the 

 ventral portion, which may be spoken of as the hyoidean vein 

 opens below into an irregular longitudinal venous sinus lying 

 just above the sucker. 



In tadpoles shortly after hatching (Fig. 78), the diverticulum 

 from the truncus arteriosus has disappeared, as has also the dorsal 

 portion of the efferent sinus ; so that the only vessels remaining 

 are the small diverticulum, eh, from the aorta, and the ventral 

 portion of the eflFerent sinus, or hyoidean vein, VY, which opens 

 below into the veins of the sucker, VK. By the time the mouth 

 opening is established, the diverticulum from the aorta has also 

 vanished, and the hyoidean vein is the only persistent part of 

 the series of hyoidean vessels. 



It thus appears that in the hyoid arch vessels are developed 

 wliich are essentially similar to those of a branchial arch ; the 

 chief difference being that no afferent lacunar vessel is formed 

 in the hyqid arch, a difference which may clearly be correlated 

 with the absence of gills, both external and internal, from the 



