2. The crab.—Obtain a crab and compare it, point by point, with the crayfish, or lobster. Notice the great width of the shell of the cephalothorax; much of this width is due to the gill-covers, which stand out from the sides of the true body. Look for the opening of the gill-chamber at the base of the pincers. Make out the stalked eyes (in sockets), the two pairs of feelers, and the five pairs of legs. Notice how the animal runs; in what respects does the method differ from the gait of the crayfish? Examine the ventral surface of a dead specimen, and notice that the abdomen is tucked under the cephalothorax. Stretch out the abdomen; compare it with that of the crayfish, and count the segments. Notice the absence of the tail fin; it is not required, as the crab does not swim. Extend the abdomen, and make drawings of the animal (i) from above, and (ii) from below, and label the parts. Carefully remove the foot-jaws and the true jaws, and compare them with [Pg 374] the corresponding appendages of the crayfish. With strong scissors cut off one of the gill-covers, and examine the gills. Observe that in the crab the gills are not in any case attached to appendages; they all spring from the sides of the body-wall. Tabulate the respects in which you have found the crayfish and crab (i) to agree with, (ii) to differ from, insects. Fig. 229.—Crayfish, seen from the right side. The appendages are numbered in Arabic and the abdominal segments in Roman numerals. ant. 1, first, and ant. 2, second, feelers of the right side; c.th., cephalothorax; E, eye; g.c., gill-cover. (× ⅔.) The student who has worked through Chapter XIX. will at once recognise in a crayfish, or a lobster, an animal possessing many features in common with the insects, and will find it interesting to try to discover for himself why it is not called an insect, but is placed by naturalists in a different class. The crayfish.—The crayfish (Fig. 229), or the lobster (which agrees closely in structure with the crayfish), is obviously an arthropod (p. 351), for it is covered by an exoskeleton of chitin, and has hollow jointed limbs and a segmented body. In these respects it agrees with the insects. On the other hand, it plainly possesses at least five pairs of legs, and has two pairs of feelers [Pg 375] (ant. 1 and ant. 2, Fig. 229), whereas no insect has more than three pairs of legs (when adult), or more than one pair of feelers. Fig. 230.—Crayfish; the right gill-chamber and gills as seen after removal of the right gill-cover. a₁, a₂, feelers; 6-13, sixth to thirteenth appendages; xiv, xv, first and second abdominal segments; E, eye; ep. 5, scoop on second maxilla; g.ch., gill-chamber; pdb. 7, gill attached to seventh appendage; pdb. 12, gill attached to twelfth appendage; plb. 13, gill on body-wall above thirteenth appendage. (Slightly reduced.)