I. CRUSTACEA: PHYLLOPODA 



365 



Sub Class II. Phyllopoda. 



The Phyllopoda are the most primitive Crustacea. The name is 

 derived from the leaf -like feet (fig. 375), which occur upon the thoracic 

 region. The anterior appendages are schizopodal, the second pair of 

 antennce often being efficient swimming organs. The number of somites 

 varies between wide linnts, there being less than a dozen in the Cladocera, 

 while, if Savigny's law (p. 352) hold true, there are over sixty in some 

 Apodidas. Most forms (the Branchipodid:e excepted) have a carapace. 

 This forms a broad oval shell covering most of the body in the Apodidas 

 (fig. 376); in the EstheriidK and Cladocera it is divided into right and 

 left halves hinged together in the mid-dorsal line, thus giving these 

 animals the appearance of bivalve molluscs. 



These forms have, besides the nauplius eye, a pair of compound eyes 

 which in the compressed forms are frequently fused, although distinct 

 in the young and retaining the double optic nerve throughout life. The 

 liver is present in the shape of simple cceca; the heart, elongate, chambered, 

 and with many ostia in the Branchiopoda, a short sac with only a pair 

 of ostia in the Cladocera (fig. 383, h), lies dorsal to the intestine. The 

 shell gland is well developed. 



In development summer and winter eggs are distinguished. The summer 

 eggs form a single polar globule and develop parthenogcnetically. The winter 

 eggs form two polar globules and require fertilization. The thin-shelled summer 

 eggs are carried by the mother in a brood pouch and soon hatch. The thick- 



FiG. 382. — Branchipus vernalis* fairy shrimp (after Packard). 



shelled winter eggs fall to the bottom, where they require a long time for develop- 

 ment. They may be dried or frozen without injury, and in some cases drying is 

 necessary for development. This explains the appearance in early spring of 

 large numbers of Branchipus mid Eslheria in pools which are dry in summer. 

 The phyllopods are largely inhabitants of fresh water. The winter eggs pre- 

 serve the species through times of drought and cold; the summer eggs are for the 

 rapid increase of the species during the wet season. The same^ relations also 

 explain the fact that males are rare and only appear at intervals, indeed are not 

 known in some species. 



