36 NATURAL HISTORY OF VERTEBRATE ANIMALS 



while coincidently the virgin forests are largely converted into 

 Sunday newspapers. 



Classification 



The Vertebrates and their Next-of-kin. — As shown by the 

 accompanying table of classification, the vertebrates belong 

 to a larger group of animals, the Phylum Chordata. All chordates 

 agree in certain fundamental points of structure, notably in the 

 possession of the notochord (cf. p. 11), as their name implies. 

 Other distinguishing features are the gill slits, and a dorsal, 

 tubular, central nervous system. The Hemichordata (Fig. 4 C) 

 are worm-Uke marine animals which are simple and yet have gill 

 slits resembling those of vertebrates. The Urochordata, or " sea 

 squirts " (Fig. 4 A), are usually modified for an attached mode of 

 life in the adult; but some are free-swimming, and in early stages 

 (Fig. 4 a) they possess a notochord and other unmistakable 

 chordate features. The Cephalochordata are represented by the 

 individuals of several genera, including Amphioxus (Fig. 4 B), 

 which do not possess a skull ; in other respects they are much like 

 the Vertebrata, which possess a skull and vertebral column, as 

 seen in familiar backboned animals (Figs. 1, 2, 3).^ Members 

 of the Phylum Chordata, therefore, exhibit a wide range of organi- 

 zation, being represented at one extreme by very lowly animals, 

 like the tunicates, and at the other by the most complex of all 

 animals, the mammals. 



Phylum, Chordata. 



Sub-phylum, Hemichordata. 



Balanoglossus, etc. (Fig, 4 C). 

 Urochordata. 



Tunicates, or sea squirts, etc. (Fig. 4 A). 

 Cephalochordata ( Acrania) . 



Amphioxus, etc. (Fig. 4 B). 



^ It is sometimes convenient to distinguish between " vertebrate " and 

 " invertebrate " animals, the vertebrates being sufficiently important, because 

 of their complexity as well as from a human standpoint, to be placed over 

 against the rest of the animal world. This distinction was originally made by 

 Aristotle. As a matter of scientific classification, however, such a division of 

 the animal kingdom into vertebrates and invertebrates is unwarranted. 



