CHAP, xiii.] THE CAT'S PLACE IN NATURE. 457 



out by the latter into a "bulbus aortoe," whence the lateral aortic 

 arches take their origin. Other transitory conditions of the cat's 

 embryonic circulation are found persistent in the cod. Thus the 

 blood is brought back to the heart by large anterior and posterior 

 cardinal veins which unite on each side in a " ductus Cuvieri," which 

 opens into the sinus venosus, and no large iliac arteries, or great 

 anterior and posterior vena3 cavaB are ever developed. Similarly in 

 the development of the cod, no umbilical veins or arteries ever 

 appear, for no allantois (and indeed no amnion) is ever formed ; but 

 the omphalo-meseraic arteries and veins are the only ones employed 

 in nourishing the embryo. But the nature of the arches which 

 support the gills needs explanation. 



We have seen how, in the cat, the hard parts of the successive 

 visceral arches of the embryo respectively become the mandible; the 

 chain of bones forming the anterior cornu of the os hyoides, and also 

 its posterior cornu or thyro-hyal.* In the cod, similar arches also 

 become (successively from before backwards), the mandible and the 

 chain of bones answering to those of the cat's anterior hyoidean 

 cornu. But the visceral arches more posteriorly placed, become 

 those successive arches which (because they support the gills) are 

 called " branchial " one branchial arch being formed from each 

 such more posteriorly-situated visceral arch. All the gill-arches of 

 each side answer, in fact, to one of the cat's thyro-hyals, as we 

 shall see more distinctly in considering Batrachians. 



The two pairs of lateral fins before noticed the pectoral and 

 ventral fins answer to the pectoral and pelvic pairs of limbs of the 

 cat, but their internal skeleton is not divisible into arm, carpus, and 

 digits ; or leg, tarsus and digits that is to say, they have not what 

 we may call the " typical differentiation'' f The muscles of the 

 limbs are few in number and simple in arrangement. The ventral 

 fins are attached to two bones which answer to the cat's pelvis, but 

 they are so far from forming a " limb-girdle " that they do not even 

 tend to reach the axial skeleton. The "shoulder- girdle," however, 

 is well developed, and, though there is no sternum, it is complete in 

 the middle line below, while above it is continued on by bones till it 

 abuts against the skull. 



The lower jaw consists of more than one bone on each side, and is 

 attached to the skull by the intervention of a complex, bony, and 

 cartilaginous structure, which may be shortly spoken of as a 

 suspeworium. Into its composition parts enter which correspond 

 to the auditory ossicles of the cat. On the base of the skull is a 

 large, long bone called the parasphenoid (which is not represented in 

 the cat's skull, save by membrane on the basis cranii), but there is no 

 basi-sphenoid and the periotic bones (i.e., the pro, epi and opisthotics) 

 remain as distinct, permanently, as they temporarily do in the 

 embryo cat. They do not, therefore, coalesce to form a " petrous 

 bone." 



* See ante, p. 339. I vertebrates which have limbs and which 



f So called because it exists in all ' are not fishes. 



