MACRURA 153 



said to place small pieces of .sand, etc., in its ears to act as 

 otoliths. Anaspides (see p. 116) is the only other Crustacean 

 which has an auditory organ in this position. 



The larval histories of the Decapods ^ 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 tliat 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 Trans- 

 actions for 1835, and described the Megalopa stage of Cancer 

 'pagurus. Eathke," although previously an opponent of Thompson, 

 subsequently made confirmatory observations upon the larvae of 

 the Anomura ; and Spence-Bate clinched the matter b}' descrilung 

 Brachyuran metamorphosis with great accuracy in ih&Pliilosopliical 

 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. 



Tliis sub - order ^ 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."* 

 Several subdivisions of the Macrura are recognised. 



' Cf. Claus, Wilrzhurgcr Xaturwiss. Zcitschr. ii., 1861, ]>. 23. 

 - Arch. f. Naturg. vi., 1840, p. 241. ^ Sj)ence Bate's Challenger Reports. 



. "* Some of the pereiopods remain l)iramous in certain Peneidea ami Caridea 

 (see p. 163). 



