THE ANIMAL KINGDOM 421 



organs are attached. The alimentary canal is a complete tube, 

 highly differentiated into various regions. The muscular system 

 is well developed; all of the muscles are striated. The nervous 

 system is of the ganglionic type, like that of annelids, but as a result 

 of condensation, or concentration, the number of ganglia in the 

 ventral chain is reduced and often does not correspond in number 

 with the body segmentation. There are two types of eyes, 

 simple ocelli and compound eyes. Except in the small crustaceans, 

 there is a blood circulatory system, but the degree of development 

 of the vessels depends upon the character of the respiratory 

 system. Thus, vessels are practically absent in forms that 

 have a diffuse organ of respiration, like the tracheal system of 

 insects, but are well developed in gill breathers, like the 

 crustaceans. The heart is tubular and lies in the mid-dorsal 

 line. Both nephridia and Malpighian tubules occur as organs 

 of excretion. 



The eggs as a rule contain a large amount of yolk, which is 

 enclosed in a thin layer of yolk-free cytoplasm. The union of the 

 male and female nuclei takes place in the center of the yolk. 

 Cleavage in large-yolked eggs is of the superficial type which 

 means that the yolk-free cytoplasm is organized into a blastoderm 

 by the immigration of cleavage nuclei arising in the yolk. Her- 

 maphroditism is rare. Alternation of amphigony with 

 parthenogenesis (heterogony) occurs. Sometimes parthenogenetic 

 reproduction in these cases takes place in the larval stage (paedo- 

 genesis). Metamorphosis is common. 



Over 600,000 species of arthropods are known, which is five- 

 sixths of the total number of known species of animals. 

 CLASS I. CRUSTACEA. Mostly aquatic, breathe by means of 

 gills, have biramous appendages, and two pairs of antennae. 

 The presence of large amounts of calcium carbonate in the 

 exoskeleton makes it thick and hard, and gives the name to the 

 group. The first two body regions are usually fused into a 

 cephalothorax. Many of the smaller Crustacea are parasitic. 

 Barnacles are the only sessile forms. 

 SUBCLASS 1. ENTOMOSTRACA. These are small in size, 

 with a variable number of segments. Head, thorax, and abdo- 

 men are distinct in some. 



Examples : Eubranchipus vernalis, the fairy shrimp ; Daphnia 

 pulex, a fresh-water cladoceran ; Temora longicornis, a copepod, 



