170 



THE KVOI.UTION TIIKOUY 



tlu'vororo greatly surjiass tlio IVoo-Hwininiin";- 1\)1)o|hii1,s in rcrdilit y, as 



is ovldi'uc'cd by the eiuinuous I'o'n'-sacs they hear at the posterior end 



ol' the body (Fig', i 13. ci). 



E\eu aiuoiio' tlie liij^her C'rustacetius, I lie so-called Malaeoslraea, 



the genuinal history not ild'reinieiitly exhibits move or less of the 



raeial history in distinet recapitulation. 



Tt is tnie lio\ve\er. as we ha\e already shown, that there are only 

 a I'ew ol' the hieju'r (^ iistaceans which t'nu>ri;'e 

 IVoni the eoo' in tlie form of a nanplins ; in 

 most of them this sta|;'e has been shnnted back- 

 wards in the ontoj^eny, and most of the era.l)s 

 and hernnt -crabs leaxc the I'j^'j;' in a hiffhor larval 

 form, that of tlie so-caJl(>d Zoavi (I'^ii;'. iH,)- 'I'his 

 term is applied to a lar\a which already exhibits 

 two main dixisions of the body, a. head a.nd 

 thorax portion (cephalothorax, Cjili) and an 

 abdomen (alid). The cephalothorax is frecpiently 

 cquippcnl with remarkable lony- s|)ines (.s/), and 

 it always bears from li\'e to eight pairs ol' lindis, 

 anteriorly tlio antennai (7 and //), then the 

 mandibles (//./), fnrtJier back swinnning-legs^ / T, 

 I'), and behind these can be recoo-ni/,eil the 

 primordia of the other legs (VrXUT), which 

 will grow freely out later on. Large facetted 

 and stalkinl eyes (An) a-re bornci on the head. 

 This Zoaia form is not now found as a mature 

 C'rusl-a.cea-n foi'iu, so wc cannot maintain with 

 any coniidt'iice that it lived as a maluie animal 

 »i an earlier period of the earth's history, but 



a.l form of tlie 



113. Tlui (nvi> 

 Hoxos of thci parnsitiu 

 Crustacean I'liimiiramn- 

 tlius gibbosux, oiilarK<'il 

 about six times ; aitor 

 C'liuis. 'I'Ik- main Hgun' 

 is Hull ol' tlio fi'iu:ili\ 

 wliuM(\ body l)onrs(iniiint 



blunt pi-ocossoH. At Its ^ Hecoiid still more wmiplex larval lorm ( 



gonital apei'ture (J) n , . , , , , . , ,. 



dwarf mule is situated, luglmr ( rustaceaiis is pn>ser\'ed lor us 111 a group 



i^andJ" thotwopairs ^c j,„^,,i,„, {1,.u,Mtaceaii,s, tlu^ Schi/,o|)ods. These 

 or appondaKi'S. «, tlio _ ' i 



long ogg-.sacs, poiiions are ( Iriistaceaiis which, though small, ap])roac.ii 

 of which liavo liccn (^it • 1 1 !■ 1 1 1^ 1 



off in the n-nin' ^^ external appearance our Iresbwater craynsli, 



only they luive, instca,d of the t-en walking-legs, 



biramose awimmiug-legs, by means of which th(\y move freely in the 



water. The number of these braiiclied legs is (ixeii greater than leu, 



thi^re are sixteen of them (Fig. 109 l>, p. 164, Vl-XIII). in the 



aiiuaria of the Zoological Sta,tioii at Naples om^ may often s<'e these. 



dainty little creatures swimming about in large companies. Here 



they are of interest to us cliietly liecause their .structure ocuMirs 



in the ontogeny of the higheist Crustaceans, the Decapods; that is, 



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