ORAL CIRRI OF SILUROIDS AND ORIGIN OP THE HEAD IN VERTEBRATES. 21 



In other Siluroids the ophthalmicus profundus may only be represented 

 by some fibres running along with the ophthalmicus superficialis of the 

 Facial 



In some Siluroids e.g. Silurtis (Stannius) and Clarias the trochlear 

 nerve runs along with the ophthalmicus profundus. That is purely a 

 case of apposition. 



The comparative anatomy of the ophthalmicus profundus is fairly 

 well known in the vertebrates, and need not be further entered into 

 here, except as regards Mijxine. In Cyclostomi, Midler and Stannius 

 showed that it contains motor fibres, a fact of which most embryo- 

 logists who have drawn up schemes of the head have been oblivious. 

 The motor fibres supply muscles attached to and working the nasal 

 tube and belonging to the tentacular system. Another feature in 

 Myxine is that a small twig from the ophthalmicus profundus supplies 

 the premaxillary tentacle. 



Premaxillary Tentacle. 



A. premaxillary tentacle occurs in Cobitidae (Misr/zirnus) and some 

 Cyprinoids e.g. Barbus, where it is attached to the premaxilla. However, 

 these forms are much modified as to histology and topographical 

 relations and are not suited to form a basis for comparison. 



In Siluroids, though the tentacle itself is absent (except in Aspre- 

 dinidae Giinther), yet there is usually present a block of precartilage 

 at the tip of the snout in relation with the premaxilla. In Callichthys 

 (Figs. 6 and 7 Pmx. p.) this block is triradiate, occupying the tip of 

 the snout below the prepalatine pieces, and on its lower surface developes 

 the premaxilla. A corresponding block is present in Chaetostomiis, 

 differing somewhat in shape. With it articulate the anterior ends of 

 the premaxillae which possess the remarkable form depicted by several 

 authors in the Hypostomidae. 



The ventral position of the mouth in these South American Silu- 

 roids is due to the large size of this piece and of the prepalatine, in 

 addition to ths presence of a rostrum. In Sturgeons it is due mainly 

 to the rostrum, while in Selachii it is brought about by the position 

 and size of the nasal capsules and the presence of rostral cartilages. 

 In embryos of various vertebrates it is due merely to the mode of 

 growth of the brain and olfactory organs. 



