LARVAE 449 



the Malacostraca as does incomplete to complete metamorphosis 

 among Insecta " (Korschelt and Heider). 



Prominent among these intercalated larval forms is the zoaea 

 larva in which (Fig. 283) the appendages of the head and of the first 

 and second (in some cases the third in addition) thoracic segments 

 are well developed, and the abdomen distinctly segmented, while 

 the posterior thoracic segments are barely differentiated and their 

 appendages absent. The larva swims by means of the exopodites 

 of the maxillipeds, and by the abdomen, and the cephalothoracic 

 shield is generally produced into long spines, which limit its 

 motion in certain directions. It is also characterized by the 

 absence of the mandibular palp. A zoaea larva of this type 

 occurs very generally among the Decapoda, and in the Euphau- 

 siidae (Fig. 291), and in a somewhat ' different form in the 

 Stomatopods (Fig. 312, p. 510). 



The stage at which free larval life is begun varies much in the 

 several groups, and, apart from the orders mentioned above in 

 which the eggs are contained in a brood space, it is generally 

 retarded most in the more differentiated members of the 

 series. It may also be retarded in species living in fresh-water 

 or on land a feature in which, as Fritz Miiller pointed out, the 

 Crustacea agree with other groups of animals.* 



In Euphausia (Thysanopodidae, Fig. 291) and Penaeus (Fig. 

 323) (Decapoda) the life-history begins, as in many Entomostraca, 

 with the naupUu^ stage, in Leucifer (Decapoda, Fig. 324) and 

 perhaps also in some Stomatopoda (see p. 508, Fig. 310, a), with 

 the metanauplius, in which, in addition to the limbs of the 

 nauphus, rudiments of the two pairs of maxillae and of the first 

 (and sometimes also the second) maxilliped, have appeared, and 

 the abdomen ends in a caudal fork. 



Sergestes, among the Penaeidea, hatches in the protozoaea 

 stage, with the limbs which are rudimentary in the meta- 



* Facts for Darwin, p. 47, London, John Murray, 1869. In this con- 

 nexion the case of Palaemonetes varians is of interest. The young of the 

 variety living in the sea, on the shores of Northern Europe (but also in 

 brackish or fresh water) are hatched as Zoaeae, in which the abdominal 

 legs are still absent, while the exclusively fresh-water forms of the S. of 

 Europe hatch in a more advanced stage, in which these legs are present 

 as biramous buds. (Cf. P. Mayer, Carcinologische Mitth. 9. Die meta- 

 morphosen v. P. varians, Mitth. Zool. stat. Neapel, Bd. ii, 1881. J. E. V. 

 Boas, Kleinere carcinol. Mitth., Zool. Jahrb. Syst., Bd. iv, 1889.) Some 

 fresh-water Caridea, however, show a contrast with their marine relatives 

 in the opposite direction, having a less abbreviated larval development. 

 Z III G G 



