462 ZOOLOGY. 



book, the various systems of any type are necessarily described 

 in comparison with the same in types previously described, 

 The student should, in the case of the Vertebrate types, re- 

 verse this, and first describe the lower of two types, and then 

 contrast the higher one with it. The questions set at the 

 end of the chapters (most of this nature being taken from 

 London University Examinations) will give the student a 

 good opportunity for testing himself. In all cases he should 

 illustrate his answers with diagrams drawn from memory 

 if only for the sake of practice in drawing. As a guide 

 we append two examples of comparisons. 



1. Compare the vertebrae of dogfish, rabbit, and frog. 



The vertebrae of the dogfish are of cartilage ; those of 

 the frog, of cartilage-bone; those of the rabbit also of 

 cartilage-bone, with epiphyses to the centra. 



The centra of the dogfish are amphicoalous (i.e. hollow at 

 either end) ; the centra of the rabbit are flat-faced, and 

 separated from one another by fibre-cartilaginous inter- 

 vertebral bodies. The centra of the frog are mostly pro- 

 ccelous (hollow in front). 



The vestiges of the notochord persist between the centra 

 in the dogfish and rabbit, within the centra in the frog. 



The transverse processes of the rabbit typically bear ribs. 

 Short ribs occur in the dogfish, but their homology with 

 those of the rabbit is doubtful. The frog has no ribs. 



The intervertebral neural plates are peculiar to the dog- 

 fish in this comparison. 



The dogfish has no zygapophyses, such as are found in 

 frog and rabbit. There are trunk- and tail- vertebrae only 

 in the dogfish ; trunk-and sacral vertebrae only in the frog ; 

 cervical, thoracic, lumbar, sacral, and caudal in the rabbit. 



2. Compare the skull of the dog with that of the frog. 



The Brain Case 



Of the frog is a cylindrical box, from which the otic 

 capsules project conspicuously on either side. It contains 

 only three ossifications in its cartilaginous substance 

 (the median sphen -ethmoid and the paired ex-occipital), 



