THE PHYLA ARTHROPODA AND ONYCHOPHORA 



for grasping and manipulating food; the development of the ehelipeds is simi- 

 lar in nature to that of the other pereiopods, but different in the extent to 

 which it has been carried. In the course of these adaptive processes, various 

 parts of the appendages have developed differently; the exopodites are often 

 reduced in size, and some disappear during the growth stages of the individual. 

 The segmental appendages of such crustaceans as the crayfish oflTer excellent 

 material for the study of serial homology. The progressive nature of the 

 modifications, which are often completed only in the adult stage, is indicative 

 of a certain amount of recapitulation; that is, each individual in its develop- 

 ment more or less repeats, in abbreviated form, the long history of evolution 

 of its ancestry. 



The gills are located in the thoracic region, lying in branchial chambers 

 covered on each side by a lateral flap of the carapace (Fig. 15.4). The gills 

 are a series of lateral outgrowths of the body wall, protected by the exten- 

 sions of the carapace, much as the gills of a pelecypod mollusk are covered 

 by the mantle flaps and the shell. In most crayfishes two kinds of gills are 

 distinguished, on the basis of their points of origin: podobranchlae, arising 

 with the non-branchial epipodites from the basal segments of the thoracic 

 limbs; and arthrobranchlae, arising from the joints by which the thoracic limbs 

 are attached to the body. In the lobster and in some crayfishes there are also 

 pleurobranchiae arising from the sides of most thoracic segments. Typically, 

 each thoracic segment bears one pair of podobranchlae and two pairs of 

 arthrobranchlae, but modifications of this plan are frequent. 



In concluding this general account of the external features of the crayfish, 

 it should be emphasized that the skeleton is a continuous structure, cover- 

 ing the entire external surface of the animal and even forming the lining of 

 the digestive tract at its anterior and posterior ends. Even the most delicate 



Ostium 



Pericardial sinus 



Heart 

 Gonad 

 Arthrobranchiae 

 Pleurobranchia' 

 Digestive gland 



Gut 

 Podobranchia 



Muscle 

 Ventral nerve cord 



Ventral thoracic artery 



Branchiopericardial 

 canal 



Afferent branchial 

 vessel 



Sternal artery 



Efferent branchial 

 vessel 



Lateral sinus 

 Sternal sinus 



Fig. 15.4. Diagrammatic cross section of a crayfish at the level of the sternal artery (cf. 

 Fig. 15.5). Arrows indicate direction of blood flow. 



429 



