442 



Mr. W. K. Parker. 



[Jan. 11, 



At first some disappointment is felt, after careful study of these 

 types, for, notwithstanding the low level in which they remain — they 

 are mere specialised Ammocostes, keeping on the same "platform" 

 as the larval Lamprey; — yet some parts of their organisation do 

 undergo a marvellous amount of transformation, and are, indeed, as 

 much specialized in conformity with their peculiar habits of life as any 

 Vertebrates whatever, the highest not excepted. 



Yet, on the whole, the Myxinoids are a sort of Ammocoetine type, 

 whilst the transformed Ammocoete, the adult Lamprey, comes 

 nearest to the un transformed Frog or Toad — the Tadpole. But the 

 mere putting of this shows (suggests at any rate) what losses the 

 Fauna of the world has sustained during the evolution of the Craniate 

 forms ; now, the Myxinoids, Petromyzoids, and numerous Amphibia 

 must all be kept " within call " of each other ; but the types that have 

 been culled out between them cannot be numbered. Some other kinds 

 of fish are evidently the descendants of primordial " Marsipobranchs," 

 notably Lepidosteus, the development of which has been lately studied, 

 and the results of which are being published in the " Philosophical 

 Transactions." But the Chimazroids, Dipnoi, and, still more important, 

 the Myxinoids, themselves, have still to be followed through their 

 early stages. If the present paper is of any value to the morpho- 

 logist, one on the embryology of these low forms would be worth 

 much more. 



The Myxinoids keep on the low " platform " of the larval Lamprey 

 {Ammocoete) in the following particulars, namely : — 



a. The notochord has no paired cartilaginous vertebral rudiments in 

 the spinal region. 



b. The trabecule end in the ethmoidal region without growing for- 

 wards into a cornu (or two continuous cornua) . 



c. There are merely "barbels" round the mouth; no labial carti- 

 lages. 



d. The last character involves this, namely, that the special armature 

 of horny teeth, attached to the labials in the adult Petromyzon, is 

 absent. 



e. The organs of vision are very feeble, and probably almost useless ; 

 in the Ammocoete they are arrested for a time. 



/. The cranium is a mere floor, without side-walls or roof. 

 The Myxinoids come near to the adult Lamprey in the following 

 particulars, namely : — 



a. There are developed outside the skull proper, but not segmented 

 from it, palato- quadrate and hyoid cartilages. 



b. There is a very large median cartilage belonging to both the hyoid 

 and branchial regions. 



c. The cranium acquires a floor by the development of a special 

 " hinder intertrabecula." 



