92 COMPARATIVE ANATOMY 



respiratory apparatus, but ' their whole organisation betrays 

 their descent from aquatic gill-breathing ancestors. 



The water-fleas and other small Crustacea nearly allied to 

 them are placed together with the barnacles in a sub-class 

 Entomostraca. As an example of this class we may take one 

 of its largest members, Apus cancriformis, an animal inhabiting 

 fresh-water pools, especially those which are dependent on the 

 rainfall and are liable to dry up in periods of drought. Apus 

 no longer exists in England, but it occurs in considerable 

 numbers in France, in South Germany, Austria, Hungary, 

 North Africa, and North America. It also occurs in Green- 

 land and Spitsbergen, so that it has a wide distribution in the 

 Northern Hemisphere. 



The life- history of the Apodidae was for a long time a 

 mystery to zoologists. After a heavy rainfall they often make 

 their appearance in large numbers in pools which have for 

 a long time been dried up, and a search in the sun-baked mud 

 at the bottom of such dried-up pools fails to reveal the presence 

 of torpid specimens from which new broods could be pro- 

 duced when the pools are refilled. Hence they were supposed, 

 by the older naturalists, to be spontaneously generated from 

 the mud and slime, and their sudden and inexplicable re- 

 appearances gave no small support to the doctrine of Abio- 

 genesis. But eventually it was discovered that the new broods 

 were simply developed from eggs deposited in the mud by the 

 old forms before the pools dried up, and, what is more curious, 

 that the eggs will not develop unless they have been dried for 

 some time. 



Mature specimens of Apus cancriformis vary in length from 

 12 to 36 mm., and may be recognised by the following 

 characters : 



The head and the greater part of the body are covered by 

 a large cephalic shield, oval in shape and evenly rounded in 

 front, but indented by a deep V-shaped notch at the hinder 

 end. Posteriorly some eleven or twelve annuli of the elongate 

 body project beyond the cephalic shield and end in a bi-lobed 

 caudal piece which bears a pair of long terminal appendages, 

 the caudal styles. The cephalic shield is divided by two 

 transverse nuchal grooves into a smaller anterior or cephalic 

 and a larger posterior region. The posterior part of the 

 cephalic shield is not attached to but only overlies the body 



