44 



THE CRUSTACEA 



n. o. 

 q. 0. 



for the purpose of attaching the animal temporarily to plants or 

 other objects. In the Notostraca, however, where the organ is 

 apparently well developed, it is not used for this purpose. In the 

 Anostraca it is absent in the adult, though very large in the larva. 

 In some of the Conchostraca (Limnadia) 

 it is prominent and pedunculated. In 

 many Cladocera it is greatly reduced or 

 0\ absent, but in some it is large and 



functional. In the Sididae the organ is 

 divided into three parts, a large median 

 anterior and a pair of smaller ones posterior 

 to it. In this case certain muscles attached 

 to the integument in the neighbourhood of 

 the organ are believed to produce a sucker- 



>,. r-r 



8 



n.lbr. 



n. md. 



n. mx, 



n.f.l. 



n.f.2. 



> n.f. 3. 



Nervous system of ' Simocephalus (Cladocera) from the 

 dorsal side. (After Cunnington.) cer, brain ; g.o, optic 

 ganglia, here partly fused in correlation with the fusion of 

 the paired eyes ; n.a', nerve to antennule ; it.a", nerves to 

 antennae, arising from the oesophageal connectives; n./.l- 

 n.f.5, nerves to the five trunk-limbs; n.lbr, nerve ring 

 encircling oesophagus in region of the labrum ; n.mrt, nerve 

 to mandibles ; n.mx, nerve to maxillulae ; n.o, optic nerves ; 

 oc, position where oesophagus passes between the nerve-cords ; 

 ventrally the oesophagus curves backwards so us to pass 

 through the ring n.lbr before reaching the mouth. The 

 transverse commissure corresponding to the antenna! ganglia 

 is on a level with the roots of the posterior antennal nerves. 



El like action, thus aiding adhesion. In some 



Lynceidae two organs are found, one behind 

 \ the other. 



n.f. 4. Endoskeletal Structures. In many 



Branchiopoda there is a considerable de- 

 velopment of trabeculae and plates of 

 tendinous connective tissue giving attach- 

 ment to muscles. This system, which 

 almost merits the name of endoskeleton, 

 is most fully developed in the Anostraca, 

 where the external cuticle remains almost 

 membranous, but it is also found in the 

 Cladocera. In the Notostraca the chief 

 part of this endoskeletal system is a 

 tendinous plate, known as the entostemite, 

 lying under the anterior part of the 

 alimentary canal and giving attachment to the adductors of the 

 mandibles and to other muscles. 



Muscular System. In the Conchostraca and Cladocera the valves 

 of the shell are drawn together by a transverse adductor muscle. 

 In some, at least, of the Cladocera this muscle is double. 



n.f. 5. 



