192 J. B. Jolinston 
of spinal nerves, at least of ventral nerves, persisting in the oceipital 
region in P. dorsatus than have been described by any author working 
on Petromyzon. It may help to elear the subject if I say at the 
outset that this fact has removed the ground upon which I was led 
to question the statement made by HaArscHek (12) and accepted by 
NEAL (27) and others that each dorsal spinal nerve belongs to the 
skin of the somite next following the myoseptum in which it lies, 
and hence is to be counted with the following motor nerve. The 
numerical relations of nerves and somites in Petromyzon as described 
by HATSCHER, NEAL and FÜRBRINGER, together with the develop- 
mental history as related by KoLtzorr, led me to think that the 
dorsal nerve may after all belong with the motor nerve next pre- 
ceding it, in conformity with HATscHer’s original statement (11) based 
upon his study of Amphioxzus and Ammocoetes. This view certainly 
seems to be supported by Amphioxus, but the conditions in P. dor- 
satus are strongly in favor of the accepted view. 
In ?. Planeri as deseribed by NEAL (28) the first three post- 
otie myotomes have no ventral nerves, but are innervated by bran- 
ches from a plexus formed by the nerves of the next two myotomes. 
In other words, myotomes 4, 5, 6, 7 and 8 (v. WIJHE) are innervated 
by the nerves of myotomes 7 and 8. According to KOLTZOFF'S account 
of the embryology of the same species, the nerve of myotome 6 is 
present in the embryo and later disappears.. In P. dorsatus the 
nerve which innervates myotome 7 (4®% post-otie) is marked in the 
accompanying plate v. 3. In front of it are two ventral nerves of 
very unequal size which form a plexus and innervate the first three 
post-otic myotomes, myotomes 4, 5 and 6. The three dorsal rami 
described innervate the dorsal parts of these myotomes which ex- 
tend forward over the ear and eye. T'he large ventral ramus divides 
into a ventral branch for the third post-otie, and an anterior branch 
for the parts of the first and second post-otie myotomes lying beneath 
the ear and eye. sSp.v. 3 and each following nerve is independent 
and innervates a single myotome. 
From this it appears that sp.v. 3 in P. dorsatus corresponds to 
sp.v. 1 of NEAL and to sp.v. 2 of KoLTZorF in P. Planeri. There 
are therefore in the Ammocoetes of P. dorsatus two ventral nerves 
anterior to those in the ammocveies of P. Planeri of about the same 
size (NEAL’s Fig. 2) and one anterior to the first nerve found in the 
embryo of the latter species. 
With regard to the dorsal nerves, NEAL and KoLTzorF agree 
