The larval histories of the Decapods[124] are of great interest, and will be given under the headings of the various groups. The first discoverer of the metamorphosis of the Decapoda was the Irish naturalist J. V. Thompson, certainly one of the ablest of British zoologists. In 1828, in his _Zoological Researches_, he describes certain Zoaeas of the Brachyura and proves that these animals are not an adult genus, as supposed, but larval forms. But Rathke, in 1829, described the direct development of the Crayfish; and Westwood, after describing the direct development of _Gecarcinus_, utterly denied Thompson’s assertions concerning metamorphosis. Thompson replied in the _Royal Society Transactions_ for 1835, and described the Megalopa stage of _Cancer pagurus_. Rathke,[125] although previously an opponent of Thompson, subsequently made confirmatory observations upon the larvae of the Anomura; and Spence Bate clinched the matter by describing Brachyuran metamorphosis with great accuracy in the _Philosophical Transactions_ for 1859. Since then a mass of work has been done on the subject, though much detail still remains to be elucidated. The Decapoda fall into three sub-orders, which graduate into one another—(i.) the Macrura, including the Lobsters, Crayfishes, Shrimps, and Prawns; (ii.) the Anomura, including the Hermit-lobsters and Hermit-crabs; and (iii.) the Brachyura or true Crabs. =Sub-Order 1. Macrura.= This sub-order[126] is characterised by the large abdomen, furnished with five pairs of biramous pleopods, and ending in a powerful tail-fan composed of the telson and the greatly expanded sixth pair of pleopods, the whole apparatus being locomotory. The second antennae are furnished with very large external scales, representing the exopodites of those appendages. Some of the Shrimps and Prawns closely resemble the “Schizopods,” but the pereiopods are nearly always uniramous.[127] Several subdivisions of the Macrura are recognised. =Tribe 1. Nephropsidea.= This tribe includes the Lobsters and Crayfishes, animals well known from their serviceableness to man. There are three families, which will be treated separately. =Fam. 1. Nephropsidae.= The podobranchs are not united with the epipodites, and the last thoracic segment is fixed and fused to the carapace. The chelae are generally asymmetrical. The most important Lobsters are the European and the American species—_Homarus vulgaris_ (= _Astacus gammarus_) and _H. americanus_ respectively; these animals engage a large number of people in the fisheries. It is estimated that in America about £150,000 are spent every year on Lobsters. The genus _Nephrops_ contains the small Norwegian lobster and other forms.