26o ZOOLOGY 



The characteristic larval stage of the Entomostraca, the 

 Nauplius, is not present in the Cladocera, except in the genus 

 Leptodora, and there it is confined to the winter-egg genera- 

 tion. 



The Cladocera are as a rule freshwater inhabitants, living 

 in enormous numbers in lakes, ponds, and springs ; only a few 

 species flourish in brackish water or in the sea. 



h. Branchiopoda. — The Branchiopoda are. Phyllopods of 

 considerable size, with clearly segmented todies often partially 

 covered hy a shield-shaped or laterally compressed shell. They 

 possess from ten to sixty pairs of foliaceous swimming append- 

 ages, which hear well-developed gills. 



The Branchiopoda differ from the Cladocera in the fact 

 that they are larger, consist of more segments, and are generally 

 more complicated in their structure. They present considerable 

 differences in the various species. Sstheria and Limnadia bear 

 large bivalved shells which completely enclose the body, Apiis 

 has a shield -shaped dorsal shell which covers the head and 

 body, but leaves the tail free, whilst Branchipus is devoid of 

 any shell. 



The flattened leaf- like appendages of Apus have been 

 looked upon as a primitive type from which the appendages of 

 other Crustacea may be derived. This type of appendage is 

 almost completely retained in the foliaceous maxillae of Astacus. 

 The abdominal appendages, that is, those which are situated 

 behind the genital openings, are the least specialised, and these 

 present an unjointed axis which bears on its inner edge six 

 processes termed endites, which bear numerous setae. The 

 axis ends in a sub-apical lobe, and carries on its outer sides 

 two exiles ; of these the distal is large, and has been termed 

 the Jldbellum, the proximal is devoid of setae, and forms the 

 branchia. 



In the female the appendage of the 11th thoracic segment, 

 which bears the opening of the oviduct, is modified ; the 6 th 

 endite and the sub-apical lobe are enlarged to form a hollow 

 cup, over which the flabellum closes like a lid, this forms a 

 receptacle for the ova, and the appendage is known as the 

 oostegopod. 



