ARTHROPODS. 217 



is brought above the alimentary canal, while the rest 

 lies below. In other words, the digestive tract passes 

 through the nervous system, a condition which is common 

 in the non-vertebrate animals, the structure in the arthro- 

 pods closely paralleling that in the annelids. 



Two kinds of eyes occur in the Arthropoda, the simple 

 or ocelli and the compound, each with an apparatus 

 (lens) to concentrate the light and a retina for its recog- 

 nition. As the name implies, the compound eye consists 

 of a number of simple eyes closely united together, the 

 whole forming a visual apparatus, each element of which 

 sees a part of the object, the whole visual impression 

 thus resembling a mosaic. 



The organs of respiration are never connected with the 

 alimentary canal, but are always developments of the 

 surface (ectoderm) of the body. They are of two kinds: 

 gills or branchiae in aquatic forms and tracheae or air- 

 tubes in the forms which do not live in the water. Gills in 

 the arthropods are outgrowths of the body-wall, usually 

 much folded or divided to afford additional surface, and 

 in these are blood-vessels. In the case of gills, then, we 

 may say that the blood is brought to the oxygen dissolved 

 in the water for that exchange of gases (carbon dioxide 

 and oxygen) upon which respiration depends. With 

 tracheae, on the other hand, the respiratory surface is 

 obtained by a forcing of the external surface into the 

 deeper parts, much as one might invert the finger of a 

 glove into the palmar region. In the tubes thus formed 

 air can enter, and thus the oxygen is brought to the 

 blood and other tissues of the body. It is interesting to 

 note that the more the tracheae are developed, the more 

 the circulatory organs are reduced. The 'lungs' of the 

 arachnid are to be regarded as modified gills. 



