258 



METAZOAN PHYLA 



There are two gill chambers, one on each side of the body, lying 

 between the lateral wall of the body and a broad plate extending ven- 

 trally from each side of the dorsal surface. Each gill chamber is open in 

 front, forming a channel in which lies the scaphognathite, or bailer, of the 

 second maxilla. It is also open by a narrow slit along the ventral side 

 and in the lower part of the posterior end. In this chamber, in the cray- 

 fishes of the eastern states, which belong to the genus Cambarus, are two 

 rows of gills. The outer row is attached to the first joints of the append- 

 ages from the second maxilliped to the fourth walking leg (Fig. 163). 

 The inner row of gills, double except in the case of the first one, arises 



InfesHne 



Digestive 

 S-homach 



\/as deferens "^^^^'^ 

 \ Heart 



"Dorsal ab- ~~ 

 \ dominal drferv 



Supraesophageal 

 gang! I on 



Anfennule 

 /infenna 



Anus 



Swirnmerets 

 Telson ,, J . - - ,.,„ , 



Ventral / Wernal 



nerve cord / artery 



Opening of 

 vas deterens 



Walking' . 

 legs 



Fig. 164. — Partly diagrammatic longitudinal section of the European crayfish, Astacui 

 fluviatilis Fabricius. {From Borradaile and Potts, "The Invertehrata," after Shipley and 

 MacBride, by the courtesy of The Macmillan Comjyany.) The indi\-idual is a male and the 

 first two swimmerets are modified to form copulatory appendages. 



from the membrane attaching the same appendages to the wall of the 

 body. In the crayfishes of the Pacific coast, which belong to the genus 

 Astacus, there is also a third row attached to the wall of the body itself. 

 In respiration a current of water is maintained in the gill chambers pro- 

 duced by movements of the swimmerets, which direct water into the 

 posterior end of the chamber, while it is being bailed or scooped out from 

 the anterior end by the scaphognathite. 



293. Internal Structure.— The various systems in the crayfish are 

 well developed (Fig. 164). Some of the systems, like the muscular and 

 nervous systems, are still metameric in their arrangement, but others 

 show much condensation. The coelom is greatly reduced in capacity and 

 becomes divided into separate cavities, including those containing the 

 reproductive organs and those about the green glands, which are excretory 



