172 LIFE BY THE SEASHORE. 



so admirably suited for the study of the characters of Crus- 

 tacea." It is commonly called a crayfish, but this name is 

 applied indiscriminately by fishermen to all the larger long- 

 tailed Crustacea except the true lobster, just as shrimp is 

 applied to the smaller forms. The true lobster does occur 

 between tide-marks, but only at low spring tides, when it 

 may be found under overhanging rocks in the deeper pools, 

 threatening the too eager naturalist with the fate which so 

 nearly overtook the Mayor of Plymouth. 



The characters of the family of Astacidae may be briefly 

 summarised as follows. The body is arched and slightly 

 compressed from side to side. The carapace has a distinct 

 cervical or neck furrow, absent in Carididae, and bears a 

 well-developed rostrum. The antennal scale is relatively 

 smaller than in the Carididae, and the antennae themselves 

 are placed beneath the antennules, not side by side with 

 them as in the higher Carididae. As in the latter, the third 

 maxillipede is elongated and leg-like. Each of the first three 

 legs ends in forceps, a condition paralleled in the Carididae 

 in the shrimp Peneus, but the first pair are much stronger 

 than the others, forming powerful weapons of offence and 

 defence. The tail is long and strong, and ends in a powerful 

 tail fan; the other abdominal appendages are more or less 

 rudimentary, but the first pair in the male are converted 

 into hardened styles. As already indicated, the Astacidae 

 stand much nearer Peneus and its allies than any of the 

 Natant genera which we have described in detail. We 

 must suppose that the higher Natantia (true shrimps and 

 prawns) and the Astacidae have both been derived, along 

 different lines, from ancestral forms which resembled 

 Peneus. 



As we have seen, the three commonest forms included 

 under Astacidae are the fresh-water crayfish, the true lobster, 

 and the Norway lobster, and it is interesting to note that 

 although they can be distinguished with perfect ease by the 

 untrained eye, yet minute scrutiny does not bring to light a 

 great number of marked differences : there is much general 

 resemblance in structure. It is also interesting to note that 

 while there are a great number of fresh-water crayfishes, 

 widely distributed over the world, there are only relatively 

 few species of Homarus arid Nephrops. 



