EXISTING ORGANISMS— CLASSIFICATION 37 



The Mollusca. The Mollusca are highly specialized, and dif- 

 ferent from the other organisms. In the class Cephalopoda, in- 

 cluding the squids, cuttle-fish, octopus, and other forms, we see 

 chiefly an illustration of different ways of obtaining the same re- 

 sult, for the blood carries oxygen, but not through the same me- 

 dium as ours, and the eyes are well developed, but different in 

 fundamental structure from ours. 



The Chordata. These organisms have departed so widely from 

 the other phyla that they stand alone. While th(>ories are avail- 

 able to explain their relationship to the remaining phyla, nothing 

 affords more exact information than the facts of general structure 

 which have been mentioned. Their isolation is so marked that 

 the term invertebrate is commonly applied to all other phyla, 

 and that of vertebrate to the better-known members of this one. 

 They are characterized by the possession of an inner stiffening 

 structure, the notochord, in contrast to the exoskeleton, or ex- 

 ternal stiffening structure developed by the skin of invertebrates; 

 the central nerve cord is dorsal instead of ventral in position; and 

 the pharynx, corresponding to the human throat, is at some stage 

 provided with a series of paired lateral openings, the gill slits or 

 pharyngeal clefts, which communicate with the exterior. The 

 vertebrates make up the greater part of the phylum, and include 

 the more familiar animals of every day experience. Six classes 

 are recognized, as follows: 



1. Cyclostomata — Round-mouthed fishes. 



2. Pisces — True fishes, with hinged jaws. 



3. Amphibia — Newts, salamanders, frogs and toads. 



4. Reptilia — Lizards, turtles, crocodiles, snakes. 



5. Aves — Birds. 



6. Mammalia — Animals which have hair, and which secrete 



milk for the nourishment of their young. 

 Relationships among the vertebrates are best emphasized in con- 

 nection with their embryological development and comparative 

 anatomy, and are of sufficient interest to us to deserve special 

 consideration in later chapters. 



The whole great field of classification involves more than 

 700,000 known species of existing animals and almost 300,000 of 

 plants. One class of Arthropoda, the Insecta, alone includes about 

 600,000 of these. Detailed knowledge of any limited group, for 

 detailed knowledge is possible only of hmited groups in the capacity 



