LARVAL CRUSTACEANS. 41 



The third part of the body, the abdomen, is trivial and shows no 

 signs of the importance it will attain to in the adult condition. No 

 limbs are present, not even a caudal fin the intestine can however 

 be traced, traversing the entire length, and opening by the anus at 

 the extreme end. A few very faint transverse markings can be made 

 out, indicating the limits of future somites. 



Now what is the significance of this most peculiar larval form ?- 



Only Scyllarus and Palinurus among the Decapods possess it. 

 Of the others, the vast majority, especially the solid phalanx of the 

 Crabs, leave the egg in a totally different condition, viz.. in the Zoe'a 

 form, i.e., with head and abdomen well developed but without thorax 

 and abdominal limbs. On the other hand the larvae of the Lobster are 

 never so primitive. Thorax and thoracic limbs are present from the 

 first though abdominal limbs are at first absent. A few others have 

 still further abbreviated metamorphosis, e.g., the Fresh-water Crayfish 

 is hatched practically in adult form, while conversely a few prawns 

 (Peneus) leave the egg in "the curiously primitive Nauplius con- 

 dition that characterises specially the Entomostracans and Cirripedes 

 and pass through numerous other stages ere reaching the adult. 



To either of the last two, Phyllosoma bears no relation, and 

 analysing the other couple, it is at once seen that the Zoe'a form is 

 also inadmissable. This one has no . thorax developed, while in 

 Phyllosoma such is of great importance. We are thus left with the 

 Lobster larva (early stage), a form at first sight nowise resembling a 

 Phyllosome. But stay ; both show distinct head, thorax and abdomen ; 



ADVANCED LARVA OF THE LOBSTER. 



in both, limbs are developed on the first two divisions, and both are 

 without appendages on the abdomen. All parts bear comparison in 

 the two forms, and the differences which, first sight, appeared so 

 great as to forbid thought of any probable fundamental similarity 

 until arrived at by analysis, are after all but comparatively trivial 

 differences of proportion. But if the larval forms of the Palinurida3 

 be so closely akin in fundamental structure (homology) to those of 

 the Lobsters (Homarus) how did the present striking divergence in 

 the form of the respective regions arise ? The answer must be 

 sought in the different habits of these animals during the larval life. 

 And the chief determining habit centres in the different mode of 



