484? Observations on the Occurrence and Non- Occurrence 



young lobster is zoeform, may be seen on a comparison of it 

 with the figure of the Z6e, p. 275. 



Mr. Thomson describes the young as " cheliferous," or 

 claw-bearing ; but this term applies as well to the adult lobster 

 as to the young one. 



" Schizopode " or split-legged. Two of the feet, at least, 

 are perfectly formed : whether the others are correctly de- 

 scribed as schizopodiform, I cannot determine. 



" A frontal spine." I could not perceive any spinous process 

 whatever about the head or thorax : the head has, I think, a 

 more larva-like appearance than any other part. The adult 

 lobster has a rostrum, or frontal spine, forming part of its 

 specific character. 



" Spatulate tail ; " that is, a tail broad and round at the 

 point, and narrow at the base. This description does not 

 materially vary from the tail of the adult. 



*' Wanting subabdominal fins." Whether the young have 

 these, I cannot determine, the imperfect developement of the 

 minor legs rendering it almost impossible to do so. 



The general appearance of the young animal is, I think, 

 much more nearly that of the perfect lobster than Mr. 

 Thompson has stated. 



{The Freshwater ', or River , Crawfish^ or Crayfish, (p. 469.)] 

 Mr. Thompson has intimated, in the letter above quoted, 

 that some peculiarity in the crawfish must have escaped M. 

 Rathke, and that he has erroneously stated that it leaves the 

 egg in a perfect state ; but subsequent observations all con- 

 firm M. Rathke's opinion. I have myself had several fine 

 specimens of the river crawfish brought to me from the 

 neighbourhood of Mattishall, Norfolk, where it is rather 

 plentiful. Several of the females had eggs, and young ones 

 just hatched under the tail. The appearance of these young 

 ones perfectly agrees with the description given by M. Rathke ; 

 no difference whatever being perceptible between their form 

 and that of the perfect animal. The eggs had tail-like ap- 

 pendages. 



On the whole, whatever differences may exist between the 

 young and adult state of some of the decapodous Crustacea, 

 it can hardly be doubted that Latreille's character, " nascunt 

 parentibus omnino fere similia," will at least apply to some 

 of them ; namely, to the mountain crab, which the Rev. L. 

 Guilding states (p. 276.) certainly leaves the egg perfect ; to 

 the Porcellana, described in p. 269, 270.; to the river craw-fish 

 (p. 469 — 484.) ; and to the common lobster (p. 482 — 484.), 

 with the qualification Latreille has given. 



On the other hand, Mr. Guilding confirms (p. 276.) Mr. 

 Thompson's statement as to the fact that some species of 



