170 



THE EVOLUTION THEORY 



therefore greatly surpass the free-swimming Copepods in fertility, as 



is evidenced by the enormous egg-sacs they bear at the posterior end 



of the bod}^ (Fig. 113, ei). 



Even among tlie higher Crustaceans, the so-called Malacostraca, 



the germinal history not infrequently exhibits more or less of the 



racial history in distinct recapitulation. 



It is true however, as we have already shown, that there are only 



a few of the higher Crustaceans which emerge 

 from tlie egg in the form of a nauplius ; in 

 most of them this stage has been shunted back- 

 wards in the ontogeny, and most of the crabs 

 and hermit-crabs leave the egg in a higher larval 

 form, that of the so-called Zoaea (Fig. 114). This 

 term is applied to a larva which already exhibits 

 two main divisions of the body, a head and 

 thorax portion (cephalothorax, Cj'jh) and an 

 abdomen (ahd). The cephalothorax is frequently 

 equipped with remarkable long spines [d), and 

 it always bears from five to eight pairs of limbs, 

 anteriorly the antennae (/ and //), then the 

 mandibles {III), further back swimming-legs {IV, 

 F), and behind these can be recognized the 

 primordia of the other legs (VI-XIII), which 

 w^ill grow freely out later on. Large facetted 

 and stalked eyes (Au) are borne on the head. 

 This Zoaea form is not now found as a mature 

 Crustacean form, so we cannot maintain with 



any confidence that it lived as a mature animal 

 is that of the female, , ,. • i r- j i n ) 1 • 1 i i 



whose body bearsquaint ^"^ ^^^ earlier period 01 the earth s history, but 



bhmt processes. At its .^ second still more complex larval form of the 



Fig. 113. Tlie two 

 sexes of the parasitic 

 Crustacean Chondracan- 

 thus gibbosus, enlarged 

 about six times ; after 

 Claus. The main figure 



genital aperture (<?) a 

 dwarf male is situated. 

 F and J", the two pairs 

 of appendages, ei, the 

 long egg-sacs, portions 

 of which have been cut 



off in the figure. 



higher Crustaceans is preserved for us in a group 

 of marine Crustaceans, the Schizopods. These 

 are Crustaceans which, though small, approach 

 in external appearance our freshwater crayfish, 

 only they have, instead of the ten walking-legs, 

 biramose swimming-legs, by means of which they move freely in the 

 water. The number of these branched legs is even greater than ten, 

 there are sixteen of them (Fig. 109 B, p. 164, YI-XIII). In the 

 aquaria of the Zoological Station at Naples one may often see these 

 dainty little creatures swimming about in large companies. Here 

 they are of interest to us chiefly because their structure occurs 

 in the ontogeny of the highest Crustaceans, the Decapods; that is. 



