THE CRUSTACEA. 45 I 



our belief, and as much might be said for the ancestry of the 

 Arachnida through the Acarida. 



The Crustacea with ten legs, and eyes which are placed on 

 the ends of short movable stalks, or peduncles, or the Decapods, 

 were the first that were found to undergo transformations. In 

 this order the head and the thorax are fused together so as to 

 form a carapace, and the branchiae or gills are attached to the 

 legs and hidden beneath it. Those decapod Crustacea which have 

 a long segmented abdomen ending in a fan-like tail, such as 

 the lobster, shrimp, and prawn, are called Macroura, and the 

 short-tailed tribe, such as the crabs, are termed BracJiyura. A 

 third order, containing the Hermit Crabs, which have a small 

 fifth pair of legs, and the abdomen more or less soft and pecu- 

 liarly formed, are the Anomoura. 



An English naturalist, Mn Vaughan Thomson, was the first 

 to recognise that the decapod Crustacea passed through a kind 

 of transformation before reaching their maturity. His obser- 

 vations were sharply criticised, but they stood the test of careful 

 examination, and some years afterwards Spence Bate's celebrated 

 essay in the "Philosophical Transactions," 1857, confirmed their 

 accuracy, and added much to the knowledge of the subject. 

 Mr. Bate remarked that previous observers and writers, including 

 Mr. Thomson, agreed that two remarkable metamorphoses take 

 place during the progressive development of the crab to the adult 

 stage. The rule holds good for most of the Crustacea, the land 

 crabs and fresh water cray-fish forming an exception. These are 

 the only Crustacea which do not undergo very decided changes of 

 form during their evolution after the escape from the e^g. 



The following is a brief analysis of Mr. Bate's communication. 

 He commences by asserting that nothing like an immobile chrysalis 

 stage occurs in the Crustacea, and that the transformations are due 

 to the more unimportant structures of one stage of life becoming 

 very important in the succeeding. The larvae of Carcinus mcsnas, 

 a common crab, were obtained : those in the earliest stages of 

 growth were procured direct from the animal, and were hatched 

 whilst in Mr. Bate's possession, and those of a later date were 

 derived from Plymouth Sound. With these specimens before him, 

 Mr. Bate traced the progressive development of the respective 



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