172 



INTRODUCTION TO ZOOLOGY 



its hinder end, and the head projecting in front. On the 

 head are the two large compound eyes which have fused 

 together to form a beautiful conspicuous structure. The 

 first antennae are rudimentary, but the second are very 

 large and branched, for they form the organs of locomotion. 



The jaws (mandibles 



and first maxillae) 

 are not visible in 

 Fig. 108, but behind 

 the head can be seen 

 the five thoracic 

 limbs of one side. 



The abdomen has 

 no appendages, and 

 terminates on each 

 side of the anus in 

 a plate bearing a 

 curved spine. The 

 alimentary canal is 

 clearly visible 

 through the trans- 

 parent body, and 

 also, lying dorsally, 

 the heart can be 

 seen, and in the 

 female the "brood- 

 pouch" in which the eggs are laid. During the summer 

 the eggs laid are not fertilised, and yet develop into new 

 individuals, an example of the phenomenon known as far- 

 thenogenesis. An egg is shown lying in the brood-pouch in 

 Fig. 108. The " winter eggs," on the other hand, are fertilised, 

 and when the skin is moulted, these eggs are cast off inside a 

 little case of hardened cuticle ; if this happens late in the year, 

 or if the conditions of life should be unfavourable, the eggs 

 rest through the winter and develop the following spring. 



Fig. 108. — Sinwcephalus, one of the Water Fleas. 



(Very like Daphnia but without the dorsal spine at the 

 end of the carapace.) 



s, Shell-gland ; h, heart ; e, egg ; 1, 1st antenna ; 2, 2nd 

 antenna ; 3-7, thoracic limbs. 



Order 2 : Copepoda 



_ , Cyclops is another minute Crustacean very 



Cyclops. ■ ^ ii.-ii, 1 



common in stagnant water ; it has a more elon- 

 gated body than Daphnia, with a more distinct, segmented 



