2o8 



THE CRUSTACEA 



Asellus and Chiridotea and more complicated in the Oniscoidea. 

 The number of types in which its structure has been studied is, 

 however, too small to admit of profitable comparisons between 

 them. In parasitic forms with suctorial mouth-parts the fore-gut 

 is probably always more or less modified, but little is known of the 

 details except in some Epicaridea. In the Entoniscidae the short 

 and narrow oesophagus leads into a sjiherical or bilobed chamber, 

 the cephalogaster, lined with villi and rhythmically contractile. 

 This is followed by a second chamber, of which tlie lumen is 



reduced to a crescentic section by a 

 strong ridge or typhlosole on the 

 dorsal side clothed with setae Avhich 

 constitute an effective strainer. A 

 third chamber with muscular walls, 

 contracting alternately with the ceplia- 

 logaster, is known as Eathke's organ. 

 In the Bopyridae the cephalogaster 

 alone has been recognised. 



Three pairs of hepatic caeca are 

 present in Lloteidae and Cymothoidae 

 (Fig. 128, hep); two pairs in Asellus, 

 Serolis, and the Oniscoidea (except 

 Ligia, which has three) ; and only 

 one pair in Gnathia, Paranthura, and 

 the Epicaridea, in which group the 

 caeca may give off diverticula. It 

 has been stated that, apart from the 

 hepatic caeca, the mid- gut of the 

 embryo gives rise to only a very 

 short region of the alimentary canal ; 

 but this has been disputed, and it 

 seems not unlikely that considerable differences may exist in this 

 respect between the members of the order. In the blood-sucking 

 Aega (Fig. 128) the stomodaeum (1) is very short, so that the 

 point of attachment of the hepatic caeca is near the anterior end 

 of the gut. It is followed by a thick-walled part (2) which 

 may possibly have a suctorial function. At about the fourth 

 thoracic somite this suddenly dilates into a thin -walled chamber 

 (3) of relatively enormous size, completely filling and distending 

 the posterior thoracic somites when filled with blood. If this 

 reservoir be really of proctodaeal origin it seems difficult to suppose 

 that it does not exercise an absorptive function. 



In some Epicaridea (Entoniscidae, Hemioniscus) the proctodaeal 

 invagination fails to unite with the anterior part of the gut and 

 either ends blindly or disappears altogether in the adult. 



Circulatory System. — The most striking features of the circulatory 



Fig. 12S. 



Acga xisora, i^ , dissected from tlip 

 dorsal side to show tlie alimentary and 

 reproductive systems. 1, stomodaeal 

 region of tlie alimentary canal ; 2, thick- 

 walled, suctorial part ; 3, thin-walled 

 reservoir ; 4, intestine ; hep, hepatic 

 caeca; ts, testis ; v.d, vas deferens. 



