THE CRUSTACEA. 451 
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 Decafods, 
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 branchie 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 MJacroura, and the 
short-tailed tribe, such as the crabs, are termed Svachyura. 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 Axomoura. 
An English naturalist, Mr. 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 egg. 
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 larve of Carcinus menas, 
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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