400 



ESSENTIALS OF ZOOLOGY 



ners of crustacean legs. They are both typically biramous and both 

 take about the same position on the body, as well as having a simi- 

 lar segmental distribution. There is also considerable similarity 

 in their structure. 



Internal Structure 



Beneath the shell-like, chitinous exoskeleton there is a very rep- 

 resentative set of systems. As in most higher animals the segmen- 

 tation is retained in the muscular system, nervous system, and to 

 a degree in the circulatory system. Earlier in the chapter it was 

 pointed out that the coelom is modified as a provision for increased 

 blood sinuses which have occupied much of the space. 



Carapace 

 removed 



Sterna/ 



artery 

 f^asclc — 



kntra/ thoracic 

 artery 



Ventral sinus 



Pericardial sinus 



heart 



Ostium 



hasc/e 



Cxonad 



Intestine 

 Digestive §land 

 Etferent /esse/ 

 GUI 



fierve cord 

 Carapace 



Vis 161 — Diagram of a cross section thirough the posterior thoracic region of a 

 crayfish. Arrows indicate the direction of flow of blood in the spaces and vessels. 



Respiratory System. — Under the branchial areas of the carapace 

 may be found the paired, feathery gills held in the gill cavity or 

 branchial chamber. There are three types of gills present here: 

 pleurobranchiae, attached to the sides of the thorax; podobranchiae, 

 arising from the epipodites of the thoracic appendages; and arthro- 

 branchiae, which arise from the coxopodites of the thoracic append- 

 ages. Several of the segments have lost the pleurobranchiae. The 

 scaphognathite moves in such a Avay over the external surface of 

 the gills as to move the water in an anterior direction. The water 

 is brought under the free edge of the branchiostegite or branchial 

 area of the carapace and moved forward to be discharged by an 

 anterior aperture. An almost constant stream of water is pumped 



