228 ELEMENTS OF COMPARATIVE ZOOLOGY. 



is without parallel in the vertebrates, but which is usually 

 met with in the non-vertebrate animals. 



The organs of respiration are never connected with the 

 alimentary canal. They are always developments of the 

 surface of the body. In the case of gills we have 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 respira- 

 tory 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 tube thus formed 

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

 and other tissues of the body. 



The Arthropoda are by far the largest group of animals, 

 the number of forms living to-day being estimated from 

 half a million to a million or more. 



The Arthropoda are subdivided into three groups or 

 "classes" : Crustacea, Acerata, and Insecta. 



CLASS L CEUSTACEA (p. 206). 

 CLASS II. ACERATA. 



In these arthropods the body is divided into two regions, 

 a cephalothorax in front and an abdomen behind. The 

 cephalo thorax bears the eyes (of which there may be several 

 pairs) and six pairs of appendages, none of which can be 

 considered as antennas. The abdomen may have or may 

 be without apparent appendages. The respiratory organs 

 are confined to the abdomen, and in their development are 

 always connected with the abdominal limbs. They may 

 be of three kinds: (1) External gills borne on the abdomi- 



