BLOOD-VESSELS OE MUSTELUS ANTARCTICUS. 
689 
it is through these that the ninth or last efferent artery {Ef. br. A. 9) pours its 
blood into the eighth ; it forms, of course, no loop, since the fifth branchial arch 
is gill-less. 
From the dorsal end of each arterial loop an epibranchial artery* (Epbr. A. 1-4) 
is continued backwards and inwards f by uniting with one another successively 
in pairs these four trunks form the dorsal aorta {D. Ao.). As seen in fig. 6 the 
first pair [Epbr. A. 1) unite in the middle line and form a short median trunk ; 
this is joined by the second pair, a thicker trunk being the result ; then the third 
pair join in like manner, and finally the fourth {Epbr. A. 4). 
The regular manner in which the epibranchials of Mustelus unite is worthy of 
notice as a clearly primitive arrangement. In the Fays (23, fig. 20), Ilolocepliali 
(Plate 36, fig. 17), and Teleostei (23, fig. 32), two or more of the epibranchials of 
the same side unite with one another before joining with those of the opposite 
side. 
In the embiyo the aortic arches are continued directly from the ventral to the 
dorsal aorta. In Ilolocepliali (Plate 36, fig. 17) and Teleostei there is only one 
efferent artery to each gill, corresponding to the anterior of the two efferent arteries 
in the plagiostome holobranch. This is very evident in Callorhynchus, in which 
the single efferent artery of each gill is always cephalad of the corresponding 
afferent trunk. These facts tend to confirm the opinion to which one is led by the 
simple inspection of the parts in the adult Mustelus (compare figs. 6 and 17) ; namely 
that the anterior efferent artery of each holobranch {Ef. br. A. 2, 4, 6, and 8 ) is 
to be looked upon as its primary revehent trunk and as strictly continuous with 
the corresponding epibranchial artery, the postei’ior efferent artery being a secondary 
vessel which debouches not into the primary trunk of its own, but into that of the 
next following gill. 
The foregoing paragraph was originally written fully a year ago, before I had seen 
Dohrn’s paper on the “Development of the Gill-Arches ” referred to above (5). It is 
satisfactory to find that the views just enunciated are largely confirmed by embryology. 
Dohrn shows that the aortic arch in Pristiurus is at first single, lying caudad of 
the head cavity. When the gill filaments are developed, accessory blood-currents 
(Nebenstrome) are set up, and give rise first to the posterior and then to the anterior 
efferent artery ; both of these open at first into the dorsal region of the aortic arch, 
which afterwards becomes the epibranchial artery. Gradually the afferent trunk, 
formed from the ventral region of the original aortic arch, loses its connexion with 
the dorsal portion, so that afferent and efferent blood-streams, communicating only by 
capillaries, are now definitely established. Then, as development goes on, the anterior 
efferent artery increases at the expense of the posterior ; much of the blood from the 
* Milne Edwakds uses this word so as to include the corresponding efferent branchial arteries, but I 
think there can hardly be two opinions as to the convenience of restricting it to that portion of the 
efferent series of vessels lying dorsad of the gill. 
MDCCCLXXXVI. 4 T 
