76 INVERTEBRATE ZOOLOGY. 



43. Draiving In this case a diagram is the only practical form of draw- 

 ing. In all such cases as in that of 40 and 41. whei'e one deals with a rap- 

 idly moving ohject. observation must consist of a long series of rapidly ac- 

 quired facts and ecjually rapid conclusions, and the drawing must he a graphic 

 representation of the facts observed. A piece of drawing paper shoiild be at 

 the side of the student, and each conclusion, when clearly formulated, should 

 be sketched in. The details of the diagram should not be all obtained from 

 one procedure ; the general shape is best gained bj^ the simple lens ; a success- 

 ful i)reparation also [42J is of great assistance in comparison with microscopic 

 observation on the living animal. 



Type VI.— DistOmum (m- allied genus). 



44. Trematodes, or fluke-worms, are as adults entloparasites in Verte- 

 brates, living in the cavities of the alimentarj- canal and its accessory organs, 

 the lungs, the gall ducts of the liver, the bladder, etc. They are almost uni- 

 versal in frogs and may be found as follows : Place a live frog under a small 

 bell jar and add a small piece of cotton saturated with chloroform. In a few 

 minutes the frog will be dead and may be removed for dissection. Place it 

 upon its back and with the scissors make a median longitudinal incision on 

 the ventral side, the entire length of the body. The shoulder girdle will be 

 met with in the thoracic region and must be also cut through. Find and ex- 

 amine the following internal organs : 



a. Lungs. A pair of dark grey sacks lying above the liver on the sides of 

 the oesophagus. If they are empty they will be small, wrinkled and very 

 elastic : if inflated they are large and conspicuous. Cut these off at their 

 base and place in a Stender dish of water for examination. 



b. Urinary bladder. This lies at the posterior end of the incision, close to 

 the bones of the hip girdle. Look for the large, dark green cloaca, or termi- 

 nus of the intestine. Upon this may be seen several wrinkled folds of trans- 

 l)arent membrane, which sjireads over quite a large area, when drawn out 

 with the forceps. This is the bladder. Trematodes may frequently be seen 

 in this, when spread out, owing to the transparency of its walls. Carefiilly 

 remove it. cutting it away from below, and place it in the dish with the lungs. 



c. Alimentary canal. This is to be cut through above the stomach and 

 just above the cloaca, and removed entire by cutting away all of its connect- 

 ing membranes, 



