374 THE CRAYFISH chap. 



3. Turn down the podobranohs and make out the 'relations of the 

 arthvobranchs from the above table. Then turn these down, or cut 

 them off, and note the single complete pleurobranch and the two 

 vestigial ones. Cut off an arthrobranch and examine its structure, noting 

 the afferent and efferent blood-vessels in its stem. Sketch. 



4. Note the branchiocardiac veins on the inner side of the thoracic 

 wall. Blow air or inject French tlue into the cut bases of the gills 

 removed, and note that the branchiocardiac trunks extend upward? tq 

 the pericardial-sinus (gee below) from the gills. 



c. General Dissection. 



Holding the animal in your left hand, insert a scalpel carefully 

 beneath the hinder edge of the carapace on the dorsal sicle, so as to 

 separate the exoskeleton from the soft integument, and then with the 

 large scissors cut alqng the outer side of each branchiocardiac groove, 

 and remove the median portion of the carapace. Note the pigmented 

 integument and then reniove it, when some of the nearly colourless blood 

 will ooze out. 



1. I. Examine a drop of blood under the microscope, adding salt 

 solution. Note the amceboid nucleated corpuscles. Sketch. 



2. The pericardial simis will now be exposed, containing the heart 

 with three pairs of valvular ostia (only the dorsal ostia can be seen at 

 present), through which the blood enters the heart from the'pericardi^l 

 sinus. 



Inject some French blue (see p. 99) into the heart through one of 

 the ostia, so as to fill the arteries (tying is unnecessary). Then remove 

 the dorsal part of the exoskeleton and integument bit by bit, all along 

 the thorax and abdomen, as well as the pair of longitudinal extensor 

 muscles lying just beneath the dorsal integument of the abdomen. Pin 

 down under water, dorsal surface uppermost, and note !^ 



3. The absence of a continuous muscular layer in the body-wall and 

 of a true coelome, and the presence of irregular spaces (blood sinuses) 

 between the viscera and muscles. 



4. The delicate arteries, arising from the anterior and posterior 

 ends of the heart : — a, the anterior median ophthalmic artery, running 

 forwards to the eye-stalks ; b, the paired anteniiary artery, on either 

 side of a, and passing forwards and downwards to. supply the gizzard, 

 renal organ, feelers, &c. ; c, the hepatic artery (also paired), rather 

 further back and more ventral, extending into and supplying the diges- 

 tive gland ; d, the median dorsal abdominal artery, arising froni the 



