146 THE CRAYFISH 



higher up and on the sides of the body, these last are termed the 

 pleurobranchiae, or "side gills." No pleurobranchise are present in the 

 adult of C. virilis. 



(f ) Make an outline ( X 2) of the cephalothorax from a side view. 

 Show the stumps of the appendages and the places from which podo- 

 branchiaj have been removed. Put in all the arthrobranchiae, and 

 show also the bailer and the epipodite. Indicate the course of the 

 water current by arrows. 



(g) Examine specimens macerated in caustic potash and notice 

 the delicate chitinous covering of the gills which has survived the 

 maceration. Are the gills inside or outside the body? In answering 

 this question, refer to Fig. 65. 



III. INTERNAL STRUCTURE 



Exercise 4. — The Digestive, Circulatory, and Reproductive Sys- 

 tems. 



(a) Using a freshly killed specimen or one that is preserved and 

 injected, cut along the dorso-lateral surface on each side of the 

 cephalothorax, taking care not to injure any of the organs lying imme- 

 diately beneath the skeleton. Remove this dorsal part of the skeleton 

 from the posterior margin of the thorax to a point just back of the 

 eyes. Cover the specimen with water in a dissecting pan. The tops 

 of the gills, which are exposed where you have cut into the gill cavity, 

 are seen on each side. The heart, which may be still beating in a 

 fresh specimen, lies on the mid-line in a cavity known as the pericar- 

 dial sinus. Although the heart is soft and spongy, you should be able 

 to distinguish the paired openings, or ostia, which are present in its 

 walls. How many are there? The anterior part of the gastric mill, 

 which is sometimes called the stomach, lies well to the front of the 

 thorax. Note its thin and delicate walls and the two transverse bars 

 of harder material, the sclerites, which are a part of the mechanism. 

 When the specimen is intact, muscles pass from each of these bars to 

 the inner surface of the dorsal skeleton. Find the remains of these 

 muscles still attached to the shell which you removed, and also to the 

 posterior sclerites. You should see the muscles arising from the ante- 

 rior sclerite and attached to the inner surface of the shell just behind 

 and between the eyes. These muscles form a part of the complex sys- 

 tem by which the grinding action of the gastric mill is accomplished. 

 Passing through the pericardial sinus are large muscles which diverge 

 as they pass forward. If these pull on their forward ends as the fixed 

 point, what movement will they bring about in the abdomen? You can 



