264 



THE OLD MOUTH AND THE NEW. 



In thecrustacea the gill usually consists of a special plate, or plume-like pro- 

 cess arising from the basal lobe of a biramus appendage. In Limulus and in the 

 eurypterids, it consists of many lamellae arising from the covered posterior basal 

 surface of the abdominal appendages. (Fig. 5.) In abnormal limulus embryos, 

 the thoracic appendages are often completely infolded, forming a leg pocket in- 

 stead of a leg process, suggesting the infolded respiratory appendages that have 

 become the normal condition in the air breathing arachnids. In the case of the 



Fig. 167. — Tadpole of frog. A, HEemal surface; JS, older stage, seen from the hgemal surface as a semi- 

 transparent object, and showing the relations of the oral, respiratory, and digestive orgas; C, same specimen 

 from the neural surface. Pigs. 164 to 167 show frog tadpoles in the ostracoderm stages. 



scorpion, the respiratory lamellae arise from an infolding just behind a small, 

 rudimentary appendage; the latter then disappears without forming a part of the 

 gill chamber. 



Without going any further into the discussion of the infinite variety of arthro- 

 pod respiratory appendages, these facts stand out clearly, namely: i. there is a 

 tendency to restrict the respiratory function to a small group of metameres, four 

 or five in number, more or less, following the oral or locomotor ones; 2. that in 



