CLASSIFICATION OF ANIMALS 175 



ing a more fundamental bilateral symmetry; three germ layers; an 

 extensive coelom which functions in circulation and respiration; a 

 water-vascular system used for locomotion and peculiar to this 

 phylum; an endoskeleton consisting of isolated plates embedded in 

 connective tissue and frequently developed as spines; a relatively 

 simple nervous system and few sense organs. 



(o) Phylum Arthropoda. — Crayfishes, lobsters, crabs, insects, 

 spiders, and scorpions, may be examined. Compare the principal 

 regions of body, metamerism, appendages, eyes, antennse, and other 

 external features. In this phylum there is great diversity of structure 

 but considerable advance in cell specialization and division of labor 

 over the annulates. Cephalization is also more conspicuous. The 

 existence of a ccelom comparable with that of annelids is problemati- 

 cal; the ancestors of arthropods may have possessed a well-developed 

 cavity of this nature. The circulation is called "open," since there is 

 a heart with outgoing vessels from which the blood enters the open 

 spaces of the tissues and so returns to the heart which it enters through 

 ostia. Other distinguishing characteristics are: a continuous exoskele- 

 ton, thinner at the joints; bilateral symmetry; metamerism; typically, 

 a pair of jointed appendages on each somite; compound eyes; three 

 germ layers. 



(p) Phylum Chordata. — ^The vertebrates, which are the conspicu- 

 ous members of this phylum, are sufficiently familiar to be excluded 

 from further description; representatives of the subdivisions of the 

 Vertebrata should be reviewed at this point by means of textbook 

 figures. Inconspicuous members of the phylum which should be 

 examined are: the lancelets, Branchiostoma (Amphioxus) ; the acorn 

 worms, Dolichoglossus (Balanoglossus) ; and the tunicates, or sea- 

 squirts. Understand the mode of life in each species studied. The 

 lancelet, or amphioxus, bears some external resemblance to simple 

 vertebrates, such as the fishes, in its shape, metameric body, fins, 

 mouth, anus, and other features. Gill slits, notochord, and a dorsal, 

 tubular, central nervous system are present internally. Examine 

 specimens or figures. The acorn worms and the tunicates suggest no 

 such relationships in their adult state. Dolichoglossus is a wormlike, 

 burrowing animal having a notochord present only in the proboscis. 

 Examine specimens or figures. Most of the tunicates are adapted for 

 a sessile life and possess inhalant and exhalant siphons that function 

 in feeding and respiration like those of a clam; indeed the tunicates 

 were classified as mollusks until the study of their development showed 

 their larval stages to possess the chordate type of organization. Speci- 

 mens and particularly figures of tunicates, showing the internal struc- 



