vI : 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 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 Trans- 
actions for 1835, and described the Megalopa stage of Cancer 
pagurus. Rathke, 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; (i1.) the Anomura, including the Hermit- 
lobsters and Hermit-crabs ; and (iii.) the Brachyura or true Crabs. 
Sub-Order 1. Macrura. 
This 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. 
1 Cf Claus, Wiirzburger Natwrwiss. Zeitschr. 1i., 1861, p. 28. 
Arch. f. Naturg. vi., 1840, p. 241. 3 Spence Bate’s Challenger Reports. 
Some of the pereiopods remain biramous in certain Peneidea and Caridea 
(see p. 163). 
Cs So = 
