638 COMPARATIVE ANATOMY 



'I he cat as an example of the mammalia. 



The frog is the classic example of the amphibian. This animal has 

 already been studied in the early part of the course, but must be kept in 

 mind so as to be compared with the above three types. 



It is usual to exclude the aves, because reptile and bird have so many 

 structural similarities that the study of one suffices for that of the other. 

 In fact the single word Sauropsida has come into common biological 

 usage as meaning both reptiles and birds. 



It is necessary first for us to have some conception of what is 

 meant by the phylum Chordata and to appreciate that there are inter- 

 mediate types between invertebrates and vertebrates. Such intermedi- 

 ate types are known as pro-chordata. The pro-chordata and the verte- 

 brata together form what Zoologists call the pyhlum Chordata. 



The vertebrates possess a spinal or vertebral column which con- 

 sists of a great number of similar portions called vertebrae arranged in 

 a longitudinal series. In the early embryo of all vertebrata there appears 

 a rod-like notochord. This probably serves as a sort of stiffening to the 

 animal and in this respect only is it similar to the spinal column proper. 

 It is neither cartilage nor bone, and probably develops from the entoderm 

 or mesoderm. As the spinal cord develops from the ectoderm, and the 

 bones of the spinal column from the mesoderm, it will be seen that 

 neither of these three just-mentioned portions are alike in either origin, 

 function, or position. 



In all vertebrates the main nerve cord lies on the dorsal side while 

 in invertebrates it lies on the ventral side. 



There are certain groups of animals which possess no spinal col- 

 umn, yet, during the embryonic period have a notochord, a dorsal nerve 

 cord, and a gill-slit apparatus (Figs. 313, 314, 315, 316). The classic 

 examples of these forms are Amphioxus, Balanoglossus, and the 

 tunicate or sea squirt, all of which are comparatively small in size and 

 live in the sea. These forms are grouped together under the name of 

 pro-chordata. Professor Patton of Dartmouth College has described a 

 scorpion in which he is sure he has found a notochord. If he is correct it 

 will be seen that no classification of this kind is absolute, in that inverte- 

 brates of very early geologic times may have possessed such an embryo- 

 logical structure. 



