CHAPTER IX 



ARTHROPODA 



This is a very large phylum of usually comparatively small animals; 

 there are more species here represented than in all the other phyla 

 combined. As their name indicates, they are characterized as the 

 first animals yet studied that have regularly jointed appendages. 

 They are like the annelids in being segmented, in having a mouth at 

 one end and anus at the other; and in having a dorsal brain and a 



ventral, ganglionated nerve cord. 

 They usually have striated muscle 

 fibres, and are almost always lacking 

 in cilia. -The body cavity is largely 

 represented by blood sinuses in free 

 communication with the vascular 

 system. The classification is exten- 

 sive but the most important classes 

 are as follows: 



I. Crustacea, Figs. 49, 50, 51, 53, 

 54 and 55. These are arthropods in 

 which four or five anterior segments 

 are fused to form the head, while the 

 rest of the segments may usually be 

 divided into two regions, the thorax 



ist antenna; a 2 , 2d an- a nd the abdomen. Some of the 

 thoracic segments may be fused with 

 the head to form a cephalothorax, as 

 in the crayfish. The head bears a 



pair of compound eyes, sometimes a median eye. The thoracic and 

 abdominal appendages are variously modified as claws, legs, fins, ac- 

 cessory reproductive organs, etc.; the typical appendage is biramous: 

 the body is covered with a variously modified exoskeleton of chitin. 

 Respiration is by means of gills or through the general body integument. 



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FIG. 49. A water flea, Cyclops. 

 Xio. 



tenna; e, eye; es, egg sac; od, ovi- 

 duct; ov, ovary; t 3 , 3d somite; ui, 

 uterus. 



