688 
PROFESSOR T. J. PARKER ON THE 
liemibranch or half-gill, and consequently nine such arteries in all (Plates 34 and 35, 
figs. 1, 2, and 6, Ef. br. A. 1-9), one for the hyoidean hemibraneh ( Ef br. A. 1) and 
two for each of the holobranchs or complete gills borne by the first four branchial 
arches. The anterior artery of each holobranch (Ef. br. A. 2, 4, 6, and 8) is markedly 
larger than the posterior. 
These vessels are usually, but very incorrectly, called branchial veins. It would be 
quite as justifiable to speak of the portal artery as to call these obviously arterial 
vessels veins ; a capillary system may be interposed in the course either of an artery 
or of a vein, but this does not make the efferent trunk in the one case a vein, nor the 
afferent trunk in the other case an artery. 
The relations of the afferent and efferent branchial arteries to one another and to 
the other structures of the gill are well seen in transverse section. Fig. 5 represents 
a section of the second holobranch taken through the dorsal end of the cerato- 
branchial. It shows the crescentic transverse section of the cerato-branchial (C. hr .) 
and the reniform section of the extra-branchial (Ex. hr.) ; one of the cartilaginous 
branchial rays (Br. r.) is cut longitudinally, as also are two branchial filaments 
(Br.Jil.) belonging to the anterior and posterior hemibranchs respectively. A muscle 
which flexes the epi- upon the cerato-branchial, and may be called the M. epi-cerato- 
branchialis ( M. ep. c. hr.), is shown in transverse section, and the great radiating 
muscle of the gill (M. rad.) in longitudinal section. It is seen that the afferent artery 
(Af. br. A. 3) lies between the radiating muscle and the branchial ray, and imme¬ 
diately external to the branchial arch ; while of the two afferent vessels, one (Ef. br. 
A. 4) is situated a short distance anterior to the radiating muscle, the other (Ef. br. 
A. 5) immediately posterior to the branchial ray. The two branches of the vagus 
(nv l , nv 2 ) are situated immediately cephalad of the radiating muscle and caudad of 
the branchial ray respectively. 
The radiating muscle may be taken as indicating the position of the head cavity 
in the embryonic gill-arch, so that, as Dohrn has shown (5), the afferent and the 
posterior efferent arteries lie caudad, the anterior efferent artery cephalad of the 
head cavity. 
The efferent branchial arteries unite with one another in a characteristic manner 
(Plate 35, fig. 6), the arteries of the anterior and posterior hemibranchs of each 
gill-pouch—not of each gill—anastomosing both dorsally and ventrally so as to 
form a complete loop. Thus the artery of the hyoidean liemibranch (Ef br. A. l) 
unites with that of the anterior hemibraneh of the first branchial arch (Ef. br. A. 2); 
the artery of the posterior liemibranch of the first branchial arch (Ef. br. A. 3) with 
that of the anterior hemibraneh of the second arch (Ef. br. A. 4), and so on. Four 
arterial loops are thus formed on each side, the ventral ends of wdiicli are united 
by a longitudinal commissure (com. 1). 
Further, the two arteries of each holobranch are placed in communication at 
about the middle of their length by two short transverse anastomoses (com. 2) : 
