452 CRUSTACEA MALACOSTRACA. 



menon which finds many parallels elsewhere. It appears to 

 be comparable with the occurrence of the tailed larva of the 

 Ascidians, of the pharyngeal clefts in the throat of the chick, 

 of the chilarian segment in the embryonic scorpion. In these 

 early stages of the life-history the larva or embryo appears to be 

 dominated by the factors which, in earlier ages, shaped its adult 

 ancestors. When we turn, however, from the mysis to other 

 and still more prevalent larval stages of the Malacostraca, 

 the zoaea and the nauplius, such an explanation appears 

 to be only very partially if at all applicable to them. The 

 zoaea larva occurs in one form or other, as we have seen, in 

 three groups of the Malacostraca, and persists in the higher 

 Decapods, notably in the crabs, in which the mysis stage is 

 absent, or, as we say, has been obliterated. The nauplius larva 

 occurs either as a free stage, or as a transient embryonic phase, 

 apparently throughout the Crustacea. Yet we have no evidence 

 of the existence, in recent or fossil forms, of a group of Mala- 

 costraca in which the posterior thoracic segments were suppressed 

 in the adult state, as they are in the zoaea, and the evidence 

 at our disposal as to the ancestral stage of the Crustacea points, 

 as we have seen, not to the three-limbed nauplius, but to a multi- 

 segmented form of the annelidan type. The explanation which 

 is usually offered for the divergence of these larval forms from a 

 phylogenetic type is that they have become adapted to some 

 special conditions of larval existence, and the parallel is drawn, 

 as we have seen, with stages of the complete metamorphosis of 

 the Insecta. Some crustacean larvae, it is supposed, remain 

 dominated by the influence of heredity, while others have struck 

 out new lines in response, now to one set of factors in their 

 environment, now to another. 



Without denying the possibility of an explanation on such 

 lines, we may point out that it must be regarded as speculative 

 until the conditions of environment have been recognized to 

 which the very diverse characters of the larvae are adapted.* 



* That the physical conditions of the mysis and the zoaea larvae 

 need not be very different is apparent from a passage (p. 362) from Mr. 

 S. I. Smith's account of the Early Stages of the American Lobster, Trans, 

 of the Connecticut Academy of Arts and Sciences, vol. ii (1873), p. 351 : 

 " The larvae (in the first Mysis stage) . . . were seen swimming rapidly 

 about at the surface of the water among great numbers of zoaea, megalops 

 and copepods." 



