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 body (Fig. 113, el). 



Even among the 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 the 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 Zosea (Fig. 114). This 

 term is applied to a larva which already exhibits 

 two main divisions of the body, a head and 

 thorax portion (cephalothorax, Cph) and an 

 abdomen (ahd). The cephalothorax is frequently 

 equipped with remarkable long spines (st), and 

 it always bears from five to eight pairs of limbs, 

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

 mandibles (///), further back swimming-legs {IV, 

 F), and behind these can be recognized the 

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

 will grow freely out later on. Large facetted 

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

 This Zoasa form is not now found as a mature 

 Crustacean form, so we cannot maintain with 



Claus. The main figure any confidence that it lived as a mature animal 

 is that of the female, . ,. • i c , i ,^>^■, i j_ 



at an earlier period or the earth s history, but 



a second still more complex larval form of the 



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 D, p. 164, VI-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, 



Fig. 113. The two 

 sexes of the parasitic 

 Crustacean Cliondracan- 

 thus gibbosus, enlarged 

 about six times ; after 



whose body bearsquaint 

 blunt processes. At its 

 genital aperture (cj) a 

 dwarf male is situated. 

 F and F', the two pairs 

 of appendages, ei, tlie 

 long egg-sacs, portions 

 of which have been cut 

 off in the figure. 



