No. 2.] AMIA CALVA. 489 



innervating them should therefore arise from or in connection 

 with sensory ectodermal thickenings as do the canal organs 

 and their nerves. Certain pharyngeal or pretrematic nerves 

 are known to innervate certain terminal buds, and the pharyn- 

 geal and pretrematic branches of all the cranial nerves arise 

 from or in connection with epibranchial or pretrematic ectoder- 

 mal thickenings (Beard, von Kupffer). The terminal buds 

 should therefore arise from or in connection with those same 

 thickenings, and as the lens of the eye belongs to the line of 

 these thickenings (von Kupffer, Piatt), it may be a modified 

 terminal bud or buds, and one of the ciliary nerves the nerve 

 innervating it. 



15. The muscles rotating the eyeball in the several orders 

 of 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 

 Cyclostomata 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 innervates 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 Elas- 

 mobranchii. Dipnoi and Urodela : in the second Ganoidei, Tele- 

 ostei, and Anura. The Amphibia are thus separated into two 

 sub-groups, corresponding to the two sub-groups of Pisces. In 

 the 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 



