DECAPODA. 527 



the females and are also distinguished by the narrower shape of 

 the abdomen (Fig. 314), as well as by the position of the open- 

 ings of the vasa deferentia on the last thoracic segment, and, 

 in the higher Decapods, by the modification of the first and 

 second pleopods as copulatory organs. In the males of the 

 Brachyura the pleopods behind the second pair are absent in 

 the adult. The remarkable effect on the gonads and secondary 

 sexual characters of Decapods caused by parasitism are described 

 on p. 415. 



In the Penaeidea the eggs are hatched at a very early stage, and 

 the females of this group have not been found carrying their 

 eggs. In all other Decapods the brood is carried by the female 

 on certain pairs of pleopods, either beneath the outstretched 

 abdomen, or between the bent abdomen and the thorax. The 

 pleopods are longer than in the males, and fringed, especially 

 the basal segments and the endopodites, with long hairs. The 

 eggs are attached by a sticky secretion to the surfaces or hairs 

 of the pleopods or to one another. The young of the fresh- 

 water crayfish have hooked ends to the blades of the chelae, 

 by which they cling without effort to the empty egg mem- 

 branes or hairs of the mother. 



The Penaeidae present peculiar structures which are apparently 

 sexual in character. In Penaeus, Leucifer and Sergestes a lobe 

 projects inwards from the base of the first pleopods of the male, 

 and may unite with its fellow to form the petasma (or curtain) 

 (Spence Bate). In the female Penaeus two lobed structures 

 have been found in connection with the 7th and 8th thoracic 

 sterna, in the middle line and projecting forwards. They con- 

 stitute the thelycum of Spence Bate. Both this and the petasma 

 are apparently variable in configuration and we have no direct 

 information as to their functions.* The larval stages of 

 Decapods are alluded to above (p. 448) and some of them are 

 figured in the sequel. 



Section 1. MACRTJRA NATANTIA. 



Body more or less compressed, abdomen well developed. 

 First abdominal segment not markedly smaller than the next. 



* The author is informed by Mr. W. T. Caiman, to whom he is indebted 

 for many valuable criticisms, that the thelycum is almost certainly an 

 external spermatheca, like that found in Cambarus and Homarus. 



