154 LIFE BY THE SEASHORE. 



detailed observation will convince you that in some respects 

 the crab and lobster resemble one another closely and differ 

 from the prawn. Thus the body of the latter is laterally 

 compressed; its dorsal shield is prolonged forward into a 

 great beak, or rostrum, which is narrow from side to side ; 

 its ten legs are placed very near the mid-ventral line, and 

 are very slender as compared with the weight of the 

 body ; its powerful tail is furnished not only with tail fins, 

 but bears also five other pairs of well-developed oar-like 

 swimmerets. clearly shown in the figure. In brief, it is 

 essentially a swimming animal, capable of supporting itself 

 in mid-water by gentle rowing movements, or darting back- 

 wards by powerful tail strokes. On the other hand, in crab 

 and tobster the body is more or less compressed from above 

 downwards; the rostrum, when present, is broad from side 

 to side ; the legs are very well developed, and are divided 

 into an anterior pair of forceps, which are weapons of 

 offence and defence, and four pairs of walking legs, which 

 are not attached at the middle of the body, but at such a 

 position as to most readily support the weight of the 

 body. In the lobster the tail is a powerful organ, but the 

 swimmerets, except the last one, are not well developed. 

 In the crab, as already seen, the tail is greatly reduced. 

 In other words, crab and lobster are typically creeping 

 animals, adapted for life on the bottom. The lobster re- 

 tains, in addition, the power of swiftly darting backwards 

 by the flexing of the tail, and therefore retains also the 

 long feelers, movable exposed eyes, and some other charac- 

 ters in common with the prawn; but the crab can only 

 crawl, and is adapted throughout for life among stones and 

 weed. 



If you have observed these points in your intact speci- 

 mens, then the next thing to be done is to take them to 

 pieces. Living specimens are best killed by dropping them 

 into very hot water for a few minutes. Of the three, the 

 Norway lobster, or crayfish, is the easiest to dissect. For 

 full details as to method, reference should be made to one 

 of the ordinary biological text-books, such as Marshall and 

 Hurst's Practical Zoology, or Thomson's Outlines of Zoology; 

 here we can only consider those points which are of im- 

 portance in our systematic survey. 



