2 NATURAL HISTORY OF VERTEBRATES. 



sense that we shall employ the teiTa, however, animals are iiicludeil with no outwanl 

 resemblance to the higher vertebrates, and, indeed, so unlike, that their affinities were 

 for a long time doubtful, till these were rendered plain by the study of development. 



All of the members of this great branch of the animal kingdom are characterized 

 by the possession of a cellular cord — the notochord — which runs underneath the 

 central nei"vous system, and which, in the higher fonus, is surrounded by the perma- 

 nent vertebral column and skull, and largely obliterated by the development of these 

 structures. So the term CHORD ATA is frequently employed as synonjTnous with 

 Vertebrata in its wide sense. In most Chordata, the anterior end of the central ner- 

 vous system is dilated into a 'brain,' and this acquires a protective capsule, the 'skull'; 

 but these structures are absent in the two lower divisions of the branch, which are 

 consequently known as Acraxia, in contradistinction to the Ceaxiata. 



Our introductory account of the structui'e of vertebrates will be confined to the 

 craniate division ; for, both as far as numbers and morjihological interest go, this is 

 overwhelmingly the most important. Of course a large amount of interest attaches 

 to the acraniate foi-ms, in view of the consideration that all vertebrates are descended 

 from an acraniate stock, but the Acraniata that have persisted to the jiresent day are 

 unquestionably not in the direct line of descent, and one of the divisions has only 

 been able to hold its own in the struggle for existence by virtue of a remarkable series 

 of degenerate specializations. The reader is referred to the special account of these 

 divisions for details with which to compare the description of the structure of the 

 craniata which follows. It need only be premised that the two divisions of acraniata 

 are distinguished from each other by the relations of the notochord. In the single 

 form, Amphioxus (Fig. 54) which constitutes the division Cephalocorda, the notochord 

 is continued as far forward as the anterior end of the head, while the Urochorda, 

 embracing all the animals known as Tunicates, pass through a tadpole-like larval stage 

 in which the notochord is confined to the tail (Fig. 47). The fact, already indicated, 

 that the Urochorda undergo a degenerate metamorphosis, their affinities with higher 

 foi-ms being chiefly observable during larval life, is sufficient to account for the much 

 greater departure exhibited by their adults than by Amiihioxus from the ordinary 

 vertebrate type. 



It will ajipear from the above what great prominence is now given by zoologists to 

 the value of affinities revealed by the study of development. To the same branch of 

 investigation are due many of the gigantic strides made by the science of comparative 

 anatomy within recent years. To enable the reader to understand the direction of 

 these, as well as to realize that embryology is often the only clue that can be relied 

 upon to guide the student through the intricacies of adult structures, it is desirable at 

 this stage to explain something of the processes by which the complex multicellular 

 body is evolved from the apjiarently so simple unicellular egg. In describing the dif- 

 ferent systems of organs which compose the vertebrate bodj-, further reference will be 

 made to the mode in which these arise from the different layers of embryonic cells, 

 even to the extent of repeating some of the statements given in the first volume of 

 this series. 



Before advancing to this part of our subject, it will be well that the reader should 

 be acquainted with some of the morphological terms employed in the description of 

 the vertebrate body. Much confusion has been occasioned by importing into compara- 

 tive anatomy terms emjiloyed in human anatomy, but the efforts of Owen, Huxley, 

 and others to introduce terms of general applicability to vertebrate animals have 



