278 THE INVERTEBRATA 



in embryonic stages of centipedes and certain insects, an appendage 

 which co-exists with the eye. 



In most compound eyes, the pigment, both in retinular and in pig- 

 ment cells, flows to and fro, being in dim light retracted towards the 

 inner or outer ends of the cells so as to leave the sides of the omma- 

 tidia exposed, and in bright light extending so as to separate the om- 

 matidia completely. In many diurnal insects it is permanently in the 

 latter position. Vision takes place in two ways according to the situa- 

 tion of the pigment. When the latter is extended, in each ommatidium 

 there falls on the retinula a narrow pencil of almost parallel rays. There 

 is then mosaic vision, an apposition image, composed of as many points 

 of light as there are ommatidia, being formed on the whole retinal 

 layer. When the pigment is retracted, each ommatidium throws a 

 complete image of the greater part of the field of vision, and the 

 images together form a superposition image, falling in such a way that 

 their corresponding parts are superposed. Superposition images are 

 less sharp than apposition images, but are formed with less loss of 

 light. Compound eyes are especially adapted for perceiving the move- 

 ments of objects, owing to the way in which such movements affect a 

 series of ommatidia in succession. 



The alimentary canal of the Arthropoda possesses at its mouth and 

 anus involutions of ectoderm, lined by cuticle, which are known re- 

 spectively as stomodaeum or fore guty and proctodaeum or hind gut. 

 These may be short, but in the higher Crustacea and Insecta form a 

 considerable part, and sometimes nearly the whole, of the canal. The 

 cuticular lining of fore and hind gut is shed at moulting. The lining 

 of the fore gut sometimes provides teeth for triturating or bristles for 

 straining the food. Digestion is extracellular, save in certain acarina. 



The respiration of aquatic arthropods, other than those which are 

 but little modified from terrestrial ancestors, is sometimes, if the 

 animal be small, effected only through the general integument of the 

 body, but usually takes place by means of gills (branchiae). These are 

 nearly always external processes, known as epipodites, which stand on 

 the bases of the limbs, and are often branched or folded. Among 

 terrestrial arthropods, some of the Arachnida possess lung books, 

 which are generally held to have arisen by the enclosure of gill books, 

 such as those on the limbs of Limulus, each within a cavity of the 

 ventral side of the body. The remainder of the terrestrial Arthropoda 

 breathe by means of tracheae, which are tubular involutions of the 

 ectoderm and cuticle which convey air to the tissues. In some 

 arachnids tracheae are present as well as lung books. Usually 

 tracheae are branched, and strengthened by a spiral thickening of 

 their chitinous lining. The study of the phylogeny of the Arthropoda 

 leads to the conclusion that a tracheal system has arisen independently 



