386 INVERTEBRATE MORPHOLOGY. 



which some of the segments behind the genital openings, 

 which may be taken as indicating the line of separation be- 

 tween the two regions, are provided with appendages. Folds 

 arising from the head region and forming either a carapace 

 or a bivalYed shell are frequently present and the animals are 

 for the most part small, the largest reaching a length of about 

 eight centimetres, while the majority measure less than a milli- 

 metre. The unpaired eye usually persists in the adult, as 

 does also the shell-gland, the antennary gland, on the other 

 hand, being usually rudimentary or absent. A masticatory 

 stomach is never present, and a further characteristic is found 

 in the fact that the larva which hatches from the egg is almost 

 invariably a Nauplius (see p. 417). 



1. Order Phyllopoda. 



The Phyllopoda are principally confined to fresh water, 

 the genus Artemia, however, being found in salt lakes, while 

 a few Cladocera are marine. They seem to be the most 

 primitive of all the Crustacea and present the greatest variation 

 in the number of metameres composing the body, some spe- 

 cies possessing over forty pairs of appendages, while in others 

 again the number is reduced to nine. All the thoracic ap- 

 pendages, however, as a rule bear branchial lobes, and in some 

 cases {Apus) present the many-lobed and imperfectly-jointed 

 condition which has been considered the most primitive form of 

 the Crustacean limb (see p. 373). The antennules are usually 

 small and abundantly provided with olfactory hairs, while 

 the antennae (except in Apus, in which they entirely disappear) 

 are long and serve as locomotor organs. The mandibles are 

 reduced to simple masticatory plates without palps, and the 

 maxillae undergo likewise considerable reduction. A heart is 

 always present, but no blood-vessels exist, the blood passing 

 from the heart into lacunar spaces. 



1. Suborder BrancJiiopoda. 



The Branchiopoda have all a plainly-segmented body con- 

 sisting of many segments, and, with the exception of BrancM- 

 pus and Artemia, are provided with a fold of the body-wall 



