XI 



PHYLUM ARTHROPODA 



561 



and a nearly complete set of appendages, all tucked in under the 

 cephalothorax and closely packed together within the egg-mem- 

 branes. In this condition the embryo is hatched, and for some 

 time clings to the pleopods of the mother by means of the peculiarly 

 hooked chela? of its first pair of legs. 



The development of the principal internal organs must be 

 referred to very briefly. From the ectoderm arise, not only the 

 epidermis of the adult, but the epithelium of the gullet and 

 gizzard and of the hind-gut, the epithelium of the gills, the 

 nervous system, the vitreous cells and retinula? of the eyes, and 



FIG. 447. Embryo of Astacus after development of thoracic appendages. A, eyes ; oj. an- 

 tennule ; no- antenna ; ab. abdomen ; </, archicerebrum and ganglion of antennule ; go, optic 

 ganglion; 1. labrum ; m. mandible; mx\. mx%. maxilla? ; t. 1 8, thoracic appendages ; 

 t. telsoii ; ts, carapace. (From Lang's Comparative Anatomy.) 



the epithelium of the auditory sac. From the endoderm arises 

 the epithelium of the mid-gut and of the digestive glands, 

 the latter being formed as tubular branching outgrowths of the 

 archenteron. The connective-tissues, the muscles, the vascular 

 system, the gonads, and perhaps the kidneys, are all of mesodermal 

 origin. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Crustacea are Arthropods in which the five anterior seg- 

 ments are fused with the prostomium to form the head, while the 

 rest are usually divisible into two regions, the thorax and the 

 abdomen. More or fewer of the thoracic segments may be fused 

 with the head to form a cephalothorax. The head may bear a 



