548 ZOOLOGY SECT. 



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 retinulae of the eyes, and 

 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 

 median eye, which frequently disappears in the adult, and a pair 

 of compound eyes, both belonging to the prostomial region : the 

 latter frequently become elevated on jointed eye-stalks. The 

 appendages of the head are (1) the antennules, which are usually 

 considered as belonging to the first metamere ; (2) the antennae, 

 which are certainly post-oral or metameric appendages shifted 

 forwards to a pre-oral position ; (3) the mandibles or crushing jaws ; 

 (4) the first maxillae ; and (5) the second maxillae. The thoracic and 

 abdominal appendages are variously modified as jaws, legs, fins, or 

 accessory reproductive organs. With the exception of the anten- 

 nules, the appendages are typically biramous, consisting of a stem 

 or protopodite bearing two branches, the endopodite and exopodite. 



The body is covered externally by a chitinous cuticle, which 

 becomes thickened and sometimes calcified in regions where no 

 movement is required, forming a series of hard parts or sclerites, 

 separated by flexible chitin : the whole chitinous cuticle thus 

 constitutes an exoskeleton. Typically there is one sclerite to each 

 metamere behind the head, and to each podomere in the appen- 

 dages, but concrescence of sclerites frequently takes place. The 

 exoskeleton is produced into setae, which are hollow processes of 

 the cuticle containing prolongations of the underlying epidermis. 



Respiration takes place either by the general surface of the 

 body or by gills, which are hollow offshoots of the thoracic wall or 

 of the thoracic or abdominal limbs. The stomodaeum and proc- 

 todaeum form a considerable portion of the enteric canal, and are 

 lined with chitin : the mesenteron gives rise to digestive glands. 

 The body-cavity is divided into compartments, most of which 

 contain blood and are portions of the vascular system : the true 

 coelome may be represented by compartments of the body-cavity 

 not containing blood and by the cavities of the reproductive organs. 

 There is a vascular system which nearly always includes a con- 



