470 Handbook of Nature-Study 



LESSON CVIII 

 THE CRAYFISH 



Leading thought The crayfish, or crawfish, as it is sometimes called, 

 has one pair of legs developed into great pincers for seizing and tearing its 

 food and for defending itself from enemies. It can live in mud or water. 

 It belongs to the same animal group as do the insects, and it is a near 

 cousin of the lobster. 



Method Place a crayfish in an aquarium (a battery jar or a two-quart 

 Mason jar) in the schoolroom, keeping it in clear water until the pupils 

 have studied its form. It will rise to explore the sides of the aquarium at 

 first, and thus show its mouth parts, legs and swimmerets. Afterwards, 

 place gravel and stone in the bottom of the aquarium, so that it can hide 

 itself in a little cavity which it will make by carrying pebbles from one 

 side. Wash the gravel well before it is put in, so that the water will be 

 unclouded and the children can watch the process of excavation. 



Observations i. What is there peculiar about the crayfish which 

 makes it difficult to pick it up? Examine one of these great front legs 

 carefully and see how wonderfully it is made. How many parts are there 

 to it? Note how each succeeding part is larger from the body to the claws. 

 Note the tips which form the nippers or chelae, as they are called. How 

 are they armed? How are the gripping edges formed to take hold of an 

 object? How wide can the nippers be opened, and how is this done? 

 Note the two segments behind the great claw and describe how they help 

 the work of the nippers. 



2. Study the pair of legs behind the great claws or chelae, and com- 

 pare the two pairs, segment by segment. How do they differ except as to 

 size? How do the nippers at the end compare with the big ones? Look 

 at the next pair of legs behind these; are they similar? How do the two 

 pairs of hind legs differ in shape from the two pairs in front of them? 



3 . Look between the great front claws and see if you can find another 

 pair of small legs. Can you see anything more behind or above these little 

 legs? 



4. When the crayfish lifts itself up against the side of the jar, study 

 its mouth. Can you see a pair of notched jaws that work sidewise? 

 Can you see two or three pairs of threadlike organs that wave back and 

 forth in and out the mouth? 



5. How many legs, in all, has the crayfish? What are the short legs 

 near the mouth used for? What are the great nippers used for? How 

 many legs does the crayfish use when walking? In what order are they 

 moved? Is the hind pair used for pushing? What use does it make of 

 the pincers on the first and second pairs of walking legs? 



6. Look at the crayfish from above ; the head and the covering of the 

 thorax are soldered together into one piece. When this occurs, the whole 

 is called a cephalothorax ; and the cover is called by the same name as the 

 upper shell of the turtle, the carapace. Can you see where the head is 

 joined to the thorax ? 



7 . Look carefully at the eyes. Describe how they are set. Can they 

 be pushed out or pulled in? Can they be moved in all directions? Of 

 what advantage is this to the crayfish? 



8 . How many antennae has the crayfish ? Describe the long ones and 

 tell how they are used . Do the two short ones on each side come from the 

 same basal segment? These little ones are called the antennules. 



