THE DRANCHIOPODA 



33 



The canipace may be directly continuous in front with the 

 dorsal integument of the head as in most Cladocera, or defined 

 from it by a groove as in some Cladocera and the Notostraca. In 

 the Conchostraca the con- 

 nection between the animal 

 and the shell is reduced to 

 a comparatively narrow neck 

 and tlie lateral lobes of the 

 shell extend forwards on each 

 side so as to enclose the 

 whole head (Fig. 17). Gener- 

 ally the shell-fold does not 

 coalesce Avith any of the 

 trunk-somites which it en- 

 velops, Avith the exception 

 of the one or two anterior 

 somites Avhich in the Clado- 

 cera are fused with the head. 

 In the aberrant Cladoceran 

 Leptodora, however, it coal- 

 esces with the dorsal surface 

 of the leg-bearing somites, 

 and its free jjortion, which 

 here, as in some other Clado- 

 cera, forms merely a brood- 



' "^ . , Fig. 18. 



sac, appears to arise from Daplmia, female, a', antennule ; o", antenna ; 



the posterior margin of the ''.<■, brood-chamber; 6r, brain; c, margin of curapace; 



, , . ^/T-i- T r.\ '••'■■' caudal setae; e, compound eyes coalesced into 



Sixth trunk-somite (rig. 19). one;/, furca; ql, maxillary gland; h. heart; hep, 



Thp pinivipp nviv l-ip liepatic diverticulum of gut ; 7i.e, nauplius eye ; oy, 



±ne carapace may oe oy^ry. (After Claus and Grobben.) 



more or less corneous, but 



it is never strongly calcified. In a few Cladocera {Monospilus, 

 etc.) and in the Conchostraca (Fig. 17) the integument of the 

 outer surface of the shell is not cast ofl:' in ecdysis, but remains 

 in position, giving rise to a series of " lines of growth " marking 

 the increased size of the shell at each moult. Special modifica- 

 tions of the carapace for protection of the eggs will be referred 

 to below. 



The number of trunk-somites varies very much. It is greatest 

 in the Notostraca, where 42 somites are found in certain 

 species of Ajyus. In the Conchostraca the number is from about 

 13 to 28, and in the Anostraca 19 to 23. In the Cladocera the 

 segmentation of the body is generally more or less obscured ; at 

 least the first two somites are always coalesced with the head. In 

 Daphnia, according to Claus, these are followed liy three limb- 

 bearing somites, and the succeeding apodous region is divided in 

 the young into three " abdominal " somites and a " postabdomen " 



