574 EVOLUTION OF ANIMALS Part V 



fers from both annelids and arthropods in being segmented only on the inside. 

 There is a pair of stiff peglegs for each internal segment. Among the annelid- 

 like structures are its thin cuticular cover, the continuous bands of muscle in 

 the body wall, and the excretory organs, a pair of coiled ciliated tubes in each 

 segment resembling the nephridia of earthworms. Arthropods lack cilia 

 altogether. The arthropodlike structures are chiefly the tracheal tubes of the 

 respiratory system carrying air directly to the tissues. A bundle of unbranched 

 tracheae extends into the body from each of the numerous external openings. 

 There is no mechanism for closing them as there is in the similar ones of 

 insects, and experiments have shown that body water evaporates through them 



Fig. 29.3. Peripatus, a walking worm. Neither an annelid worm nor an ar- 

 thropod yet resembling each, it has internal segments like the worms and air tubes 

 like the insects and spiders. This connecting link is distributed in regions of the 

 Southern Hemisphere. (Courtesy, Pauli: The World of Life. Boston, Houghton 

 Mifflin Co., 1949.) 



about 40 times more rapidly than in a caterpillar. The skin of Peripatus is 

 adapted to moist land life and is restricted to it. The unguarded holes are un- 

 safe against the evaporation of dry air. The advancement of Peripatus has 

 doubtless been hindered by too much ventilation. 



Trilobites — The Pioneer Arthropods. Over half the fossils that date from 

 the first era ef invertebrates are trilobites (Fig. 29.4). They were arthropods 

 with 3-lobed bodies and many pairs of uniform 2-branched appendages, the 

 latter probably for locomotion. In the course of time new types of arthropods 

 developed from certain of the trilobites. The sea scorpions were among those 

 that became the ancestors of the spider tribe (arachnids) and the horseshoe 

 crabs (Limulus) which have survived into the present day. The trilobites, once 

 the most numerous of invertebrates, now exist only as fossils but their 

 descendants have more than taken their places (Fig. 29.1). 



Class Crustacea 



With but few exceptions, crustaceans are a great tribe of animals that 

 breathe by gills. Some have pioneered into fresh water and a few live cau- 

 tiously on land but, like their ancestors, most of them belong in the sea. There 

 they exist in untold numbers and in great variety of shapes and sizes. Through 

 all its variations the crustacean plan is evident — the segmented body bearing 

 jointed appendages that typically have two branches (Figs. 29.5, 29.6). Crus- 

 taceans range in size from water fleas that are microsopic, and barnacles an 



