196 FOURTH REPORT 1834. 



into the two legions of Parasites Marcheurs and Parasites 

 Nagetirs, the former comprising the single order Araneiformes, 

 the latter the two orders Siphonostomes and Lemeens. The 

 third • suhclass consists of the single order Xyjihosures. It will 

 be seen that Edwards has adopted a large number of Latreille's 

 principal groups. At the same time he has introduced some 

 changes in the arrangement of this author. He has augmented 

 the nmnber of orders, and likewise altered the limits of some of 

 these divisions. Two of the additional orders are for the recep- 

 tion of the Pt/cnogonida and LerncecB, w^hich Latreille does not 

 include in the present class. In the descriptive portion of his 

 work, M. Edwards has as yet pnjceeded but a little way. In 

 fact he has only got through the first two families of the Decw 

 poda Brachyura. A few years back, however, he published a 

 monograph on the Crustacea Amphipoda, to which those may 

 be referred who want information on that particular order. An 

 extract from it will be found in the Ann. des Scien. for 1830*. 



Some researches on the Crustacea by a naturalist of this 

 country, of great importance, though leading to results which it 

 would be w'ell to have confirmed by other observers, may be 

 noticed in this place. I allude to Mr. Thompson's supposed 

 discovery of a metamorphosis in the animals of this class, an- 

 nounced in 1828, in the first number of his Zoological Re- 

 searckesf. It is stated by this gentleman, that having examined 

 the newly hatched young of the common Crab ( Cancer Pagujnts), 

 he found them presenting exactly the appearance of the Zoea 

 Taurus, the Monocuhis Taurus of Slabber, which animal he 

 conceives to be the first state of the species above mentioned. 

 From this circumstance he was led to conclude, that metamor- 

 phosis was general throughout the Decapod Crustacea ; that in 

 the first stage of their existence they are essentially natatory, 

 but that after a time the greater number of them lose the power 

 of swimming, acquire chelae, and have their feet adapted for 

 crawling only. In a communication made by letter to the Zoo- 

 logical Society in 1830:}:, Mr. Thompson stated, in support of 

 the universality of this metamorphosis, that he had ascertained 

 the newly hatched animal to be a Zoea in eight genera of the 

 DecupodaBrachyura,\\z. Cancer, Carcinus, Portimus, Mryphiay 



• torn. XX. 



t Zoological Researches, and Illustrations ; or Natural History of Nondescript 

 or imperfectly known Animals. By J. V. Thompson. Cork, 1828, &c. — Of this 

 work only five numbers have as yet appeared. In it will be found some other 

 valuable memoirs relating to the Crustacea besides that above alluded to, more 

 particularly one on the genus Mysis, and another on the Shixopoda. 



X Proceed, of Zool. Soc, p. 17, 



