448 On the Skeleto7i of the Marsipobranch Fishes. [Jan. 18, 



stage. The various sections of the adult were made from the larger 

 river species, P. fluviatilis, so also were the various larval specimens ; 

 but the embryos were of the small kind — P. planesi. These were 

 reared by Messrs. Salvin and Balfour. 



I have first described the skeletal structures after metamorphosis, as 

 their condition then is best known to anatomists. I then explain 

 what is seen in the embryo, and after this the larval or ammoccetine 

 stage. If my friends are successful in obtaining for me larval 

 Lampreys actually transforming, a Third Part, a much smaller paper, 

 will be prepared. 



In spite of the invaluable help I have received from my fellow- 

 workers, my task has not been an easy one ; it has been taken up 

 again and again, after research into the development of other fishy 

 types. 



The suctorial mouth has its highest development in the Lamprey ; 

 in the Myxinoids (Myxine and Bdello stoma) there is no circular disk 

 with horny teeth, but merely an oral fissure, surrounded by barbels, 

 and having inside it a huge tongue beset with two oblique rows of 

 recurved and inturned horny teeth, antagonised by a single ethmoidal 

 tooth, in the larva of the Lamprey the mouth is not circular, and the 

 lower lip is far back, covered by the upper, which is like a hood ; 

 there are no teeth of any kind, only moss-like " barbels " or papillae 

 under the upper lip. 



In the Tadpole the mouth is suctorial, the lower lip being converted 

 into an imperfect ring, which is completed by the upper lip. Here the 

 cartilage of the lower lip is not a perfect ring, as in the Lamprey, but 

 is in two parts, and is formed into a sort of horseshoe. Inside this 

 compound ring there are sharp horny plates or teeth, and the folds of 

 the lips, all round the mouth, are covered with a horny rasp. 



Correlated with the perfectly suctorial lower lip of the Lamprey, 

 which is & post-oral structure entirely, we have the most perfect form 

 of the superficial branchial skeleton, a basket-work of soft cartilage, 

 which appears in the early embryo, and only gains enlargement, fore 

 and aft, with all its snags and outgrowths, after metamorphosis. Besides 

 this there are no rudiments of internal branchial arches, such as we 

 find in the Tadpole. The only parts developed inside the head- 

 cavities and branchial arches are the generalised and rudimentary 

 mandibular and hyoid arches. In the Tadpole there is no pier to the 

 hyoid arch, and the first cleft is arrested as a small blind pouch ; this 

 state is persistent in the Lamprey. But, after metamorphosis — the 

 liugering latter part of that profound change of structure — the 

 young Frog and Toad acquire a pier to their hyoid arch, right and 

 left. This, however, does not become functional to the arch, much 

 less assist in supporting the mandible, as a " hyomandibular," but is 

 transformed into an osseo-cartilaginous chain — a stapedio-incudal 



