THE ELASMOBRANCH FISHES 



175 



the first pocket, and the posterior efiferent-collector (tlie tliird) drains the 

 area in front of the second pocket. The anterior etferent-collector, by its con- 

 nection through cross-trnnks (c.tr., fig. 161) with the posterior collector back 

 of it and by its ventral continuation with the collector in front of it, receives a 

 considerable amount of blood and is a much larger vessel than is the posterior 

 eflferent-collector of the same gill. It is the anterior efferent-collector which is 

 continued directly outward to the dorsal aorta as the branchial efferent proper 

 (see fig. 161, hr.ef.). All the efferent-collector loops posterior to the one just 

 described are similarly made up of posterior and anterior efferent-collectors, 

 and all are emptied into the unpaired dorsal aorta similarly through branchial 

 efferents (hr.ef.) which are the direct continuation of the anterior efferent- 

 collectors. 



d.a'- 



Fig. 163. The developing branchial arteries, Squalus acanthias. (From Scammon.) 



a.a.^''^, first and sixth embryonic aortic arches; af., first afferent artery; cl., gill cleft; 

 d.a.^, paired dorsal aorta; d.a., dorsal aorta; ef., efferent artery; e/c", anterior efferent- 

 collector; efo.b, posterior efferent-collector; ps., pseudobranchial artery; sp. spiracle; v. a., 

 ventral aorta; x., where break in primary arch will take place. 



We may briefly consider the formation of the afferent and efferent systems 

 in the embryo of Squalus as described by Scammon (1911), The arterial 

 system here consists of (1) a ventral aorta (v. a., fig. 163) running forward 

 from the heart under the gill region, (2) a dorsal aorta (d.a.) extending 

 backward above this and dorsal in position, and (3) six aortic arches (a.a.'^~^) 

 connecting dorsal and ventral aortae in the pharyngeal or branchial region. 

 Upon the formation of the gill filaments a new branch (e/c") is budded off 

 which collects the blood from the newly formed gill tissues. The origin of this 

 collector from the embryonic aortic arch marks the place where the embryonic 

 arch later separates (see last arch at x) into two parts, the upper becoming 

 the efferent (ef.) which joins the dorsal aorta, and the lower the afferent 

 (a.f.), which joins the ventral aorta. The collector (e/c") (efferent-collector) 

 next sends cross-trunks backward to the posterior demibranch and a posterior 

 efferent-collector (efc.^) is formed. In the last arch it will be observed that the 

 posterior efferent-collector has not as yet formed. In a general way it may be 

 said that for every embryonic aortic arch, except the first and second, two ef- 

 ferent-collectors thus arise. One of them is formed for the anterior demibranch 

 (efc") , the other for the posterior (efc.^). The two collectors then continue to 

 grow downward and the tip of the posterior efferent-collector now joins the 



