M. Coste on the Habits of some Marine Animals. 201 



out in irregular plates upon a surface of considerable extent ; in 

 others, such as the Prawns and Shrimps, the semen is emitted 

 in the form of spermatophora, which are attached to the plas- 

 tron or to the base of the feet of the females. In these species, 

 therefore, the spermatozoids must become disaggregated exter- 

 nally to reach the oviducts, without their being introduced there 

 by the males. But the fecundation is not less ovarian than in 

 the preceding, although it approaches more nearly to the period 

 of oviposition. There are even species in which it only precedes 

 the expulsion of the eggs by a few hours. Thus, for example, 

 we have seen a Prawn (Palcemon serratus, Leach) copulate on one 

 day and deposit its eggs on the next. 



As all the Crustacea carry their eggs under the tail, or upon 

 some other part of the body where the incubation takes place, 

 and as this incubation is generally very slow (it does not last 

 less than five or six months in the Lobsters and Palinuri), it 

 follows that the animals of this class, in casting their shells, 

 would have been liable to lose their progeny, unless, by an ad- 

 mirable combination, the periods of the oviposition had not been 

 calculated in relation to those of the moults to which these spe- 

 cies are annually subjected until they have acquired their defi- 

 nitive size. Thus, to obtain all the time necessary for incubation, 

 nature has chosen that fecundation should take place immediately 

 after the mothers have thrown off their old skins, in order that 

 the new ones may be a sufficiently lasting protection. 



We have seen a striking proof of this fact in the common 

 Shore Crab and the Prawn of our coasts. The male of the 

 former species selects a female, holds her tightly with the right 

 foot of the second pair, carries her with him, whether he walks 

 or swims, and seizes her again if they be separated. Some days 

 after this union, the female, still held by the male, throws off 

 her old covering ; and immediately after this moult is effected, 

 she turns to receive the semen, — an operation which lasts one, 

 two, or three days. 



In the Prawn (Palcemon serratus, Leach), the male does not 

 take possession of the female before the moult, as in the com- 

 mon Shore Crab ; but as soon as she has moulted, he pursues 

 her, darts upon her back, clings there, and allows himself to be 

 carried about without making any attempt at copulation as long 

 as the female swims ; when she stops, he glides under her by 

 inverting the right side, deposits a double spermatophore upon 

 her plastron in a few seconds, and then resumes his former posi- 

 tion, to recommence the same manoeuvre a moment afterwards. 



There are some species which have two broods between 

 each moult, and in which the copulation fecundating the first 

 generation appears also to fecundate the second; of this w 



