292 THE CRUSTACEA 



G. Smith in the males of some Oxyrhyncha, and it is very probable 

 that it may occur also in other groups of Decapoda. 



Mention may be made here of the remarkable phenomena of 

 "parasitic castration," discovered by Giard in Decapods infested by 

 Khizocephala, Entoniscidae, and other parasites, and more recently 

 investigated by G. Smith. The latter -observer finds that, in 

 Brachyura infected with Sacculina, the females show very little 

 modification of external characters beyond a reduction in size of 

 the pleopods, although the gonad is reduced in size or even 

 completely eradicated. Infected males, however, assume in various 

 degrees the secondary sexual characters proper to the female ; the 

 chelipeds (in species with dimorphic males) remain in the form of 

 the non- breeding phase and resemble those of the female ; the 

 abdomen becomes more or less broadened and may assume com- 

 pletely the female form ; the copulatory styles (first and second 

 pleopods) are greatly reduced, and small pleopods may appear on 

 the third to the fifth abdominal somites. In the most completely 

 modified specimens only the reduced copulatory styles remain to 

 show that they once were males. The very remarkable observation 

 has been made that these completely modified males, in the rare 

 cases when they recover sufficiently from the parasitic infection 

 to regenerate a gonad, become perfect hermaphrodites, the gonad 

 producing both spermatozoa and ova. 



Observations, as yet unpublished, made by A. Wollebaek, 

 seem to indicate that certain deep-water Decapoda are normally 

 hermaphrodite. 



DEVELOPMENT. 



With some noteworthy exceptions to be mentioned below, the 

 Decapoda pass through a more or less extensive metamorphosis 

 after leaving the egg. The most complete series of changes occurs 

 among the Penaeidea, some at least of which are hatched as free- 

 swimming nauplii and have a larval history closely parallel to that 

 of the Euphausiacea. 



In the Penaeidae the development was first made known 

 by Fritz Miiller, and further elucidated by Clans, Brooks, and 

 Kishinouye. The Nauplius (Fig. 170, A), which has been hatched 

 from the egg by the last-named of these authors, has a quite typical 

 form. The pear-shaped or oval body is 'without a shell-fold and 

 has two terminal setae posteriorly. The median eye is present and 

 the three pairs of nauplius-limbs, the third pair of which are with- 

 out any masticatory process. In the succeeding Metanauplius-stage 

 four pairs of limb-rudiments are developed behind those already 

 present, the masticatory process appears on the third pair, the 

 swimming -branches of which begin to diminish, and a pair of 

 papillae on the anterior margin represent the " frontal organs," 



