128 ARTHROPODA. 



Order 8. Hymenoptera. 



Metamorphosis indirect. Two pairs of membra- 

 nous wings. Mouth-parts suctorial and man- 

 dibulate. (Apis, Vespa.) 



CRUSTACEA. 



HOMARUS AMERICANUS. (Lobster.i) 



These animals are not generally found where they can be 

 readily observed in nature, but many valuable observations can 

 be made on specimens confined in aquaria. If other animals 

 are present in the aquarium notice the position of defense that 

 is taken. In nature the animal probably spends much of its 

 time under rocks with the anterior end of the body turned to- 

 ward the opening. In this position both sense organs and 

 weapons are in the proper position for attack or defense. Notice 

 how the appendages are used. Are the sense organs moved fre- 

 quently? Why are the eyes on stalks? What appendages are 

 used in walking? Are all of these appendages used in just the 

 same way ? Does the animal move equally well in all directions ? 

 Perhaps you can make the animal swim; if so, observe the 

 method. Feed a specimen with portions of a clam or fish, and 

 see how food is torn to pieces and transferred to the mouth, and 

 determine, if possible, how the mouth appendages are used. 



External Anatomy. — As in Nereis, the body is segmented. 

 The metameres of the head and thorax, however, are immova- 

 bly fused to form a cephalo-thorax. This is covered dorsally by 

 a single piece, the carapace. 



1. Note, on the carapace, the cervical groove between the head 

 and thorax, and the beak or rostrum, forming an anterior spine. 

 The ventro-lateral edge of the carapace is not attached. A flat 

 object thrust between it and the body passes into the gill cham- 

 ber. This free plate of the carapace is called the gill-cover. 

 Do you know wh}^ the edge of the carapace is free? Notice the 



^ These directions may be used for the crayfish without much modifi- 

 cation. The smaller size of these animals will make it more difficult to 

 trace some of the nerves and blood-vessels 



