80 ZOOLOGY. 



Class I. — Ceustacea {Water-fleas, Shrimps, Lobsters, 

 and Crabs). 



General Characters of Crustaceans. — The typical forms 

 of this class are the craw-fish, lobster, and crab, which the 

 student should carefully examine, as from them a general 

 knowledge of the class, which varies greatly in form in the 

 diSerent orders, may be obtained. The following account 

 of the lobster will serve quite as well for the craw-fish, 

 which abounds in the rivers and streams of the Central and 

 Western States. 



The lobster's body consists of segments, six of which in the abdo- 

 men are seen to form a complete ring, bearing a pair of jointed ap- 

 pendages. Tlie abdomen consists of seven segments. One of these 

 segments (Fig. 94, I)') should be sep- 

 arated from the others by tlie student, 

 in order to observe the mode of inser- 

 tion of the legs. Each segment bears 

 but a single pair of appendages, and it 

 is a general rule that in the Arthropods 

 eacli segment bears but a single pair 



of appendages. Tlie abdominal feet 



,,■,,, . , „ J. Fig, 95 — Mandible of the lobster, 



are called swimmerets; they are Homarus americanus: pal. 

 narrow, slender, divided at the end palpus, 

 into two or three lobes or portions, 



and are used for swimming, as well as in the female for carrying 

 the eggs. The first pair are slender in the female (Fig. 94, Bj) and 

 not divided, while in the male (Fig. 94, B s) they are much larger, 

 and aid iu reproduction. The si.xth segment (Fig. 94, (?) bears broad 

 paddle-like appendages, while the seventh segment, forming the end 

 of the body and called the "telson," bears no appendages. It repre- 

 sents the tergum alone of the segment. Turning now to the cephalo- 

 thorax, we see that there are two pairs of antennae, the smaller pair 

 the most anterior; a pair of mandiiiles with a palpus, situated on 

 each side of the mouth; two pairs of ma.xillff,' or accessory jaws, 

 which are flat, divided into lobes, and of unequal size; three pairs of 

 foot-jaws (maxillipedes), which differ from the maxilla? in having 

 gills like th(jse in the five following pairs of legs.* There are thus 



* The stiKlents can separate these hnit)a in a bcdled lobster or cray-flsh, and 

 compare thein with the cuts. He will find the exercise an interesting one. 



