326 
EDWARD PHELPS ALLIS. 
pouch, separated from it by the branchial and extrabran chial 
rays of their arch. The fibres that form the distal edge of 
the muscle, as they cross the dorsal and ventral edges of the 
next posterior gill-pouch, are , strongly attached by connective 
tissues to those edges, and ventral to the gill opening a 
certain number of them unite to form a larger strand, which 
then forms the distal edge of the muscle. By far the larger 
part of the muscle strands of the dorsal half of the con- 
trictor traverse the branchial septum, only a few of them, 
one to three strands, being inserted on the epibranehial of 
the related arch. In the ventral half of the muscle, on the 
contrary, a considerable number of strands have their origins 
from the related ceratobranchial, and the additional strands 
having this origin apparently correspond to those fibres of 
the dorsal half of the muscle that have been utilised to form 
the musculi interarcuales dorsales II and III of Dohrn’s 
descriptions, these muscles being represented in Scy Ilium by 
a single muscle, the musculus arcualis dorsalis. A few strands 
of the constrictor have their origins, dorsally, on the dorsal 
extrabran chial of their arch, near the dorsal bend in the 
extrabranchial, and a somewhat larger number of strands 
are inserted, ventrally, on the ventral extrabranchial of the 
arch, near its ventral bend. All of thejfibres of the muscle 
that lie distal to those thus inserted on the extrabranchials 
cross the external (anterior) surface of both the dorsal and 
the ventral extrabranchials and, as shown in the figures, 
are attached ventrally, by connective tissue, to the external 
surface of the longitudinal hypobranchial muscles, none of 
them reaching the mid-ventral line. 
Proximal (anterior) to the fibres that have their origins on 
the dorsal extrabranchial a few strands of the muscle in each 
arch have their origins in loose connective tissues of the 
region, and proximal to the fibres that are inserted on the 
ventral extrabranchial quite a number of strands unite to 
form a muscle bundle which, as just above stated, corresponds 
to the musculus arcualis at the dorsal end of the arch. These 
ventral strands have a different course and insertion in each 
