ALIMENTARY CANAL. EXCBETORY OUGANS. 409 



and eyes with smooth cornea (Claaocera). The former possess 

 numerous lenses, and are sometimes placed on movable stalks 

 (Decapoda). Occasionally accessory eyes are found on other parts 

 of the body, on the jaws and between the legs of the abdomen 

 (Eupliausia). 



Auditory organs are found most frequently in the Crustacea as 

 auditory vesicles with otoliths in the basal joint of the anterior 

 antennae, or rarely in the appendage of the abdomen known as the 

 fan (tail of Mysis). In Insecta, auditory organs of a very different 

 structure have been discovered. 



Olfactory organs are also widely distributed. They are situated 

 on the surface of the antennae, and consist of delicate tubes or 

 peculiar conical projections, beneath which the sense nerves end in 

 ganglionic swellings. 



Tactile organs. The antennae and palps of the oral appendages 

 and the ends of the limbs have a tactile function. These parts are 

 provided with peculiar hairs and seta?, beneath which nerves end in 

 ganglionic swellings. 



Alimentary canal. An independent digestive apparatus is always 

 present, but its structure and degree of development are very 

 various. The alimentary canal is only exceptionally degenerated 

 and absent (Rhizocephala). The mouth is placed on the ventral 

 surface of the head. It is furnished with a projecting upper lip, 

 and usually with paired appendages, which are used either for 

 masticating or for piercing and sucking. A narrow or wide 

 oasophagus leads into the intestine, which either simply traverses the 

 axis of the body or is disposed in several coils. The oasophagus and 

 midgut (chyle stomach) may even be divided into several regions, 

 and may possess salivary glands and hepatic appendages of various 

 size. 



Excretory organs. Urinary organs are widely distributed. In 

 the simplest form they appear as cells on the surface of the intestine 

 (lower Crustacea), in a more highly developed state as tubular 

 filiform diverticula of the hindgut (Malpighian tubes) (fig. 329). In 

 the Crustacea, glands are present in the shell (shell glands') and in 

 the base of the posterior antenna*; they are regarded as the 

 morphological equivalents of segmental organs. 



The circulatory and respiratory organs present the greatest 

 differences in the various groups of the Arthropoda. In the 

 simplest case the clear, more rarely coloured blood fluid, which is 

 often corpusculated, fills the body cavity and the interstices of all 



