1883.] On the Skeleton of the Marsipobranch Fishes. 441 



the skull-base, where other types show but little or only temporary 

 distinctness of parts, these fishes develop and retain large indepen- 

 dent cartilages. 



The lamprey has a large superficial basket-work of soft cartilage 

 (extra-branchiaV) , and its gill-pouches keep related to this and to the 

 rest of the structures of the mouth and throat. But in the Myxinoids 

 the basket-work is iutra-branchial, and corresponds to the system of 

 segmented arches of the higher Cartilaginous, the Ganoid, and the 

 Osseous fishes. But these non-segmented arches soon lose all relation 

 to the branchial pouches, which are removed so far backwards that 

 they begin under the twentieth myotome; whilst the end of the peri- 

 cardium is under the fortieth. 



In seeking light upon the primordial condition of the Vertebrata, one 

 naturally looks to such forms as the Myxinoids. For in these types, 

 even in the adult state, there are neither limbs nor vertebras, and no 

 distinction between head and body, except the beginning, in the head, of 

 a cartilaginous skull — a continuous structure — not showing the least sign 

 of secondary segmentation, and by far the greater part of it in front 

 of the notochord, or axis of the organism. But here our gradational 

 work agrees with the developmental, for the continuous skull-bars con- 

 stantly arise before the secondary cartilaginous segments that are found 

 between the myotomes behind the head. Evidently, therefore, the early 

 " Craniata" grew supports to the enlarged and subdivided front end 

 of their neural axis, long before any structures beyond strong fibrous 

 septa were developed between the muscular segments of the body. 

 As for the linear growth, the greater or less extension backwards of 

 the main organs — circulatory, respiratory, digestive, urogenital — that, 

 in the evolution of the primary form, was a thing to be determined 

 by the " surroundings " of the type. " Thereafter as they may be " 

 was the tentative idea in this case. 



Certainly, in the Marsipobranchs, and in their relations, the larval 

 " Anura," we have the most archaic " Craniata " now existing ; in 

 these the organs may be extended far backwards in a vermiform 

 creature, as in these low fishes, or kept well swung beneath the head 

 — the body and tail together forming merely a propelling organ, as 

 seen in Tadpoles, especially the gigantic Tadpole of Pseudis. 



Thus we see that in low limbless types there is no necessity for the 

 development of more than fibrous " metameres " in the spinal region ; 

 but the vesicular brain, the suctorial lips, the branchial pouches, and 

 the special organs of sense — these all call for support from some 

 tissue more dense than a mere fibrous mat or web. In the Myxinoids 

 we see that four special modifications of the connective tissue series 

 are developed for the support of the properly cephalic organs, and for 

 them only ; thus these fishes are Craniata, but are not Vertebrata; 

 that is, if we stick to the letter, which, of course, we do not. 



