220 PROFESSOR MARSHALL AND EDWARD J. BLES. 



loops, G C, connecting the afferent and efferent vessels are diagram- 

 matically expressed. The gill in its true proportions is seen in 

 Figs. 9 and 10. 



d. The Second Branchial Arch. — In this arch the vessels have 

 essentially the same arrangement as in the first branchial arch, and 

 there is no occasion to describe them in detail. The efferent vessel 

 is prolonged ventralwards along the anterior border of the afferent 

 vessel, but, as in the first arch, it ends blindly, and there is no com- 

 munication between the afferent and efferent vessels of the arch 

 except through the gill capillaries. 



e. The Third Branchial Arch. — From the posterior border of the 

 afferent vessel of the second arch, some distance beyond the point 

 where it leaves the vessel of the first arch, but before it reaches the 

 gill, a short afferent diverticulum (Figs. 7 and 10, A F.3) is given 

 off, which enters the third branchial arch, and then ends blindly. 



In the arch itself, at the side of the pharynx, there is an efferent 

 vessel of considerable size (Fig. 7, E F'"), which is widest opposite 

 the small external gill, and is prolonged upwards above this point to 

 open into the dorsal aorta. 



Opening into the efferent vessel opposite the gill is a small 

 afferent vessel which is continued downwards in the arch, lying 

 immediately behind the efferent vessel, but not extending so far 

 ventralwards. 



The condition of the external gill of the third branchial arch is 

 somewhat variable in specimens of this size. When fairly well 

 developed, as in Fig. 9, it contains a capillary loop connecting the 

 afferent and efferent vessels together. 



/. The Fourth Branchial Arch. — There is as yet no afferent diverti- 

 culum of the truncus arteriosus for this arch. An efferent vessel of 

 some size is present in the upper part of the arch (Fig. 7, E ¥""), 

 which is prolonged upwards and forwards to join the dorsal aorta, 

 just behind the point of opening of the efferent vessel of the third 

 arch. From the widest part of the fourth efferent vessel a small but 

 distinct pulmonary artery arises (Figs. 7 and 9, A P), which runs 

 backwards and inwards to the rudimentary lung. 



Behind the efferent vessel, and ventral to the origin of the 

 pulmonary artery, a small lacunar space is present (Fig. 7, A F""), 

 which is apparently the first trace of the afferent vessel of the fourth 



