PHYLUM ARTHROPODA 



661 



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 embrj'o is hatched, and for some 

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

 hooked chelae 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 viti'eous cells and retinuLTe of the eyes, and 



Fig. 447. — Embryo of Astacus .ifter development of thoracic append.if?c.s. A, eyes; a-], an- 

 tcnnule ; a^. antenna ; ab. abdomen ; rr, ai'chicerebrum and ganglion of antennule ; r/o, optic 

 ganglion; /. laljrum ; i». mandlTilc ; mx^. mj:2. maxillae; t. 1 — 5', thoracic appendages; 

 t. telson ; 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 Characteks 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 fu.sed 

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



