No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 741 
the Ichthyopsida are not homologous structures, if existing 
descriptions of their innervation can be relied upon. On the 
contrary, the group Ichthyopsida (the Pharyngobranchii ex- 
cluded) can be divided by the innervation of the muscles of 
the eye into two great groups and other sub-groups, which, if 
reversions have not taken place, indicate distinct and definite 
lines of descent. 
In one of the two great groups, represented by the Cyclosto- 
mata alone, the nervus abducens innervates two recti muscles, 
the inferior and the externus. In the other group that nerve 
innervates but one rectus muscle, the externus, but it inner- 
vates also a retractor bulbi when that muscle is found. 
The second group is subdivided into two sub-groups, in one 
of which the superior branch of the oculomotorius, lying 
dorsal to the ophthalmicus profundus, innervates two recti 
muscles, the superior and the internus, while in the other it 
innervates but one, the superior. In the first sub-group are 
found Elasmobranchii, Dipnoi, and Urodela; in the second, 
Ganoidei, Teleostei, and Anura. The Amphibia are thus 
separated into two sub-groups, corresponding to the two sub- 
groups of Pisces. Inthe prototype of this second group there 
must have been an arrangement of muscles and nerves most 
nearly represented by that found in the Holocephala, where 
the rectus internus arises near the front end of the orbit, as in 
Petromyzon, and the obliquus superior from the edge of the 
orbit, as in Petromyzon. From this prototype two lines lead 
to the two sub-groups of Pisces and two to the two sub-groups 
of Amphibia. 
Between the two lines leading to Amphibia lies Ichthyophis, 
in which the muscles rotating the eyeball are innervated as in 
Anura, but in which a retractor tentaculi has been formed 
from one of the muscles of the eye (Sarasins), probably from 
the rectus internus of Urodela, and not from the retractor 
bulbi, as the Sarasins suggest. Ichthyophis seems therefore, 
in the arrangement of the muscles of the eyeball, to represent 
the beginning of the line leading to higher vertebrates, as 
Burckhardt states that it does in the arrangement of the parts 
of the brain. 
