82 



THE CRUSTACEA 



body. In the Chondracantliidae the anterior two pairs alone are 

 developed, and these become enlarged in the adult into clumsy, 

 unsegmented bifid lobes. In other cases the thoracic limbs are 

 reduced to minute, unsegmented processes, or some or all of them 

 may disappear. In a few cases the adult is entirely without 

 appendages, as in the Herpyllobiidae. 



Alimentary System. — The alimentary canal is in many cases of 

 simple form, not divided into sharply defined regions and Avithout 

 diverticula. The stomodaeum and proctodaeum ax'e short. In 

 many Gymnoplea there is a short median diverticulum anteriorly, 

 and in some cases, immediately behind this, a pair of small lateral 

 (hepatic) caeca which may be bifid {Eucalanibs). In some Cory- 

 caeidae and Asterocheridae these caeca are large and much 

 branched (Fig. 46). Groups of gland-cells described as salivary 



occur in the region of the 

 labrum and epistome. 



The extrinsic muscles of 

 the alimentary tract are well 

 developed in Eucopepoda. 

 Certain muscles running 

 from the anterior part of the 

 gut to the dorsal and anterior 

 region of the bodj^-wall are 

 of importance in producing a 

 rhythmical displacement of 

 the whole alimentary canal, 

 and serving, in the absence 

 of a heart, to cause a circu- 

 latory movement of the blood. 

 In some parasites the dilator 

 muscles of the oesophageal 

 region are greatly developed 

 act as a suctorial 

 apparatus. The short rectum 

 (proctodaeum) is usually pro- 

 vided with dilator muscles running outwards to the body-wall 

 in addition to the usual constrictors, and the rhythmical movements 

 of dilatation and contraction produced by them have been regarded 

 as subservient to a process of anal respiration. 



The alimentary canal is usually nearly straight (except for the 

 sternal flexure of the oesophageal portion), but in a few Gymnojilea 

 and in Cancerilla (Asterocheridae) its course is slightly sinuous. 



Circulatory System. — A heart is present in most Gymnoplea and 

 in the genus Misophria among the Podoplea. In all other Cope- 

 poda it appears to be wanting. When present it has an abbreviated 

 saccular form, and is situated in the region of the first or second 



Fig. 46. 



Artotrogus orbicularis. Outline of body, from and 

 above, showing ramified lateral diverticula of the 

 alimentary canal. (After Giesbrecht.) 



