664 



FRESH-WATER BIOLOGY 



Fig. 1013 



Estheria morsei, with left valve of shell removed. X Q. 

 ai, first antenna; 02, second antenna; c. carapace; d, cercopods 

 or furcal rami; /, flabella; u, umbone. (After Packard.) 



The alimentary canal of phyllopods consists of a large mastica- 

 tory and glandular atrium produced by an overhanging labrum 



c in front of the mouth; 



this is followed by a 

 buccal cavity, a ver- 

 tical esophagus and a 

 small globular stom- 

 ach within the head; 

 and, behind these, is a 

 long straight intestine 

 which terminates in a 

 short rectum at the 

 posterior end of the 

 body. The heart is 

 greatly elongated in 

 the Anostraca, oc- 

 cupying nearly all the trunk-somites, with a pair of ostia opening 

 in each somite. In the Notostraca and Conchostraca it is more 

 restricted — and extends through only three or four segments in 

 the latter. There are no definite blood vessels. A maxiUary gland 

 (consisting of an end-sac, glandular coiled tube, and short terminal 

 duct) serves as an excretory organ in phyllopods. The ladder-like 

 structure of the ventral nerve chain shows the primitive character 

 of the nervous system. 



After leaving the egg, all American 

 phyllopods begin their development as a 

 free swimming nauplius or metanauplius 

 (Fig. 1014) . Some differences exist even 

 in closely allied forms in regard to the 

 stage of development reached at hatching. 

 The larvae of the Notostraca and Anos- 

 traca are typical metanauplei at the time 

 of hatching, with an oval body that shows the beginning of several 

 trunk-somites posteriorly and sometimes the rudiments of their 

 appendages. The first antennae are well developed but uniramous, 

 the second antennae have a movable masticatory process and the 

 mandibles are but feebly developed. The earliest conchostracan 



Ftg. 1014. Metanauplius of Apus can- 

 criformis, just hatched. (After Claus, 

 from Lankester's Treatise on Zoology.) 



