Noss) WOSCLES AND NERVES TN AMIA, CALVA: 679 
erly, immediately median to the third muscle, a branch of the 
ramus pharyngeus of the next following arch runs downward 
to the pharynx. 
In Heptanchus the interarcuales are found much as they are 
in Acanthias ; in Scymnus interarcuale I is entirely wanting. 
In all three fishes the muscles are said, by Vetter (No. 124, p. 
443), to be innervated by pharyngeal branches of the vagus 
nerve that belongs to the corresponding branchial cleft ; hence, 
necessarily, the nerve of the next posterior cleft, for the glosso- 
pharyngeus belongs to the cleft in front of the first arch. He, 
however, only succeeded in definitely tracing the nerves to the 
muscles in one fish, Scymnus. The corresponding muscles in 
Mustelus and the rays are first said by Tiesing, in a somewhat 
uncertain way (No. 123, pp. 111 and 115), to be innervated by 
the nerve of the arch to which they belong. This statement 
is then qualified as to the innervation of interarcuale I on the 
first arch of Mustelus, but not as to the corresponding muscle 
in the other fishes. Fiirbringer refers to this qualification of 
the first positive statement regarding Mustelus, as confirmation 
of his own statement (No. 36, p. 133) that the interarcuales I 
are all innervated by branches of the spino-occipital or spinal 
nerves, and the interarcuales II and III, on each arch, by the 
nerve of the arch to which they belong. 
There are thus three different innervations given for these 
muscles. If Vetter were right, each vagus nerve would have 
to innervate muscles found on the arch in front of its cleft as 
well as others found on the arch behind that cleft. This is 
directly opposed to what van Wijhe finds in larvae of Scyllium 
and Pristiurus, the posttrematic branch alone of each branchial 
nerve, in those fishes, innervating the muscles of the arch of 
the segment to which the nerve belongs (No. 130, p. 1g). If 
the branchial clefts were intrasegmental and the arches, there- 
fore, intersegmental, as Hoffmann says they are in Acanthias 
(No. 57, p. 647), the innervation given by Vetter would seem 
possible or even natural. The clefts are, however, said to be 
intersegmental, and the arches, therefore, intrasegmental in 
Amphioxus (No. 54, p. 145), in Ammocoetes (No. 54, p. 159), 
and in Necturus (No. 91, p. 955). The innervation given by 
