The oral cirri of Siluroids and the origin of the head in Vertebrates. 381 



Technique. 



The models have been made after the wellknown method of Born, 

 l)ut with certain additions to the technique. The heads of the small 

 hsh were decalcitied in picric acid , stained with alum carmine, and 

 subsequently with bleu de Lyon. Camera drawings were made on 

 ordinary tracing paper and tlicn rolled in with the wax to make the 

 ])lates. 1 used the cheap impure wax with a low melting point of 

 the kind used, I am told, by printers. A large rectangular museum 

 l)ottle i)roved the best table on which to cut out the plates. 



After reaching a certain size the models became extremely diffi- 

 cult to handle and it was necessary to find some method of strength- 

 ening them. I decided to electroplate them. The parts were wired 

 together and then l)rushed with a suitable kind of blacklead. Then 

 by applying in the copper sulphate bath, a current up to 5 or 10 

 ampères, according to the size of the model, a deposit of copper of 

 sufficient solidity was formed in a few hours. 



Subsequently they were painted and photographed. For oppor- 

 tunity of carrying out these operations I am indebted to the Physical 

 Department of the Owens College. 



By subsequent calculation 1 find that the models are an appre- 

 ciable fraction too long in comparison with the breadth, but that does 

 not detract to any great extent from their value. The wax plates 

 must be made considerably thinner than the calculated thickness, but 

 how much thinner depends on the "personal equation". 



On the occurrence of oral cirri. 



Oral cirri, barbels, barbules- or tentacles occur in many fish, and 

 form one of the characters diagnostic of the Siluroids (Nematognathi), 

 in which they occur throughout, with the apparent exception of 

 Plecostomtis. Much information on them can be gained from syste- 

 matic works, especially from Günther's Catalogue of Fishes, where 

 the suliject is treated from the systematist's point of view. 



True barbels do not grow out indefinitely, but only with certain 

 morphological relationships, and the maximum number in the Crauiata 

 is 6, or perhaps 7 pairs, which I term nasal, premaxillary, maxillary, 

 coronoid, mental (and extramental) and submandibular. 



I attempted to draw up a list showing the occurrence of the 

 individual tentacles, but had to relinquish it from lack of morpho- 

 logically i)recise observations. These tentacles may be bifid and per- 

 haps completely split, or fringed, but nevertheless all processes from 



Zool. Jahrb. VIII. Abth f. Morph. 25 



