THE CRAYFISH 



145 



the legs and see how the outer gills arc related to them. How many 

 outer gills are there? To what appendages are they attached? What 

 effect do you think the animal's walking has upon respiration? What 

 structure do you find at the place where you saw the flickering move- 

 ment under the shell of the living specimen? Back of this bailer is 

 another, more delicate, blade which is the epipodite of the first 

 maxilliped. 



(b) From what you have observed about the gill cavity, its con- 

 tents, and the water currents you have seen in the vicinity of a quiet 

 animal, explain how the gills are bathed with a constantly changing 

 supply of water. 



(c) The outer gills are called podobranchiae ; note the significance 

 of the name. Keeping the specimen entirely under water, and lifting 



carapace 



ostium 



heart 



gonad- 

 arthroijronch/ae 

 pJeuroOranch/a 



digestive ^/anc( 



gut 

 podot)ranch/a 



mu£>c/e 

 ver>tra/ nerire cord 



s>ubneura/ artery 



pericardial sinus 



branch io - pericard/a/ 

 cana/ 



afferent fyrancfyja/ 

 i/esaef 



sterna/ artery 



efferent lyranch/af 

 i^esse/ 



/atera/ sinus 

 ventra/ sinus 



Fig. 65. — The thoracic region of the crayfish, in cross section; the arrows 



show the direction of blood flow. 



the podobranchiae one at a time to be sure you do not destroy any of 

 the smaller gills which lie close beneath, remove all the podobranchise 

 by cutting them off close to the appendages. Cut one across the 

 middle with scissors and examine the section under water with a hand- 

 lens. The afferent and efferent branchial vessels will be seen cut 

 transversely. 



(d) The inner layer of gills is now exposed. Are they attached to 

 the legs? There are five pairs and a single one in front. Opposite 

 which of the appendages are these gills located? They are called the 

 arthrobranchiae, or "joint gills." Note again the significance of the 

 name. 



(e) In the lobster and in some crayfishes there is another layer of 

 gills lying beneath the arthrobranchiaj. Because they are attached 



