l68 THE ORIGIN OF VERTEBRATES. 



most cases of alteration. The author claims to have discovered 

 in the king-crab a large auditory sense-organ akin to the pecten 

 of the scorpions, which is the original of the vertebrate auditory 

 apparatus. 



The history is carefully followed of the legs and other ap- 

 pendages, the leg-jaws and masticatory apparatus of the king- 

 crab and sea-scorpion ; and the author shows apparently conclu- 

 sively that they have been metamorphosed into the tentacles and 

 upper and lower lips which surround the mouth of the larval 

 lamprey. In the case of the Silurian fishes, such as the 

 Ptcrichthys, he demonstrates that they have followed a different 

 line of development in the paddles, or flappers, of the Ganoid 

 fishes. 



Dr. Gaskell points out also the likelihood that some of the 

 earlier Silurian fishes were, not only without the usual jaws 

 characteristic of the vertebrate skull, but were, on the other 

 hand, provided with appendages less developed, perhaps, but 

 resembling the legs and leg-iaws ( endognaths and ectognaths) 

 of the arthropods, thus constituting an important link between 

 the arthropoda and vertebrata. Furthermore, the head-shields 

 of the Silurian and Devonian fishes, Pteraspis, Cyathaspis, etc., 

 not only greatly resemble the carapace of some of the arthropods, 

 but are composed of muco-cartilage like the muco-cartilaginous 

 head-shield of the larval lamprey, and not of bone, as is usual in 

 vertebrate skeletons. 



The cartilaginous skeleton of the larval lamprey, it may be 

 said, is entirely different in substance from the bony skeleton 

 of the vertebrate, while it is very similar to that of the king- 

 crab, and presumably to that of the fossil forms. 



In other respects it is shown that the difference is greater 

 between the larval lamprey (ai)iiiwca'tcs)an(\ the adult form 

 ipctromyzon) than it is between the king-crab (liniulus), or 

 .'■■ea-scorpion (curypterus) and ammoca:tcs. 



Indeed, the most reiuarkable feature in the whole history 

 of this singular development is that the final stages of the meta- 

 morphosis are accomplished, not in the transition from one 

 allied species to another — not even in the issue of the earliest 

 and most primitive vertebrate from the latest highly developed 

 invertebrate — by what might almost be called the normal pro- 

 cess of fertilisation, gestation and birth, but in the lifetime, and 

 by the metamorphosis, of the individuals of closely allied species, 

 or varieties, during the short period that was requisite in order 

 to complete the growth or transformation from larva to adult. 



Were it not for the support afforded by this singular fact, 

 constituting, perhaps. Dr. Gaskell's strongest argument, it would 

 be difficult to accept his explanation of the formation of the 

 vertebrate mid-gut, and its introduction between the original 

 cephalic and thoracic region (pro- and meso- soma) and the 

 original cloacal region Oneta-soma). For it evidently owes its 

 inspiration to the necessity of increasing the length of the body 



