416 



ARTHttOPODA. 



optic nerve throughout life. The liver is present in the shape of 

 simple ca3ca; the heart, elongate, chambered, and with many ostia 

 in the Branchiopoda, a short sac with only a pair of ostia in the 

 Cladocera (fig. 420, U), lies dorsal to the intestine. The shell 

 gland is well developed. 



In development summer and winter eggs are distinguished. The sum- 

 mer eggs form a single polar globule and develop partheuogenetically. 

 The winter eggs form two polar globules and require fertilization. The 

 thin-shelled summer eggs are carried about by the mother in a brood 

 pouch and hatch in a relatively short time. The thick-shelled winter eggs 

 are cast off and fall to the bottom, where they require a long time for 

 development. They may be dried or frozen without injury, and at least in 

 some cases drying is necessary to their development. This feature explains 

 the appearance in early spring of large numbers of Branchipus and 

 Estheria in snow pools which are dry throughout the summer. On our 

 western plains and in Europe Apus occurs in the same way. These pecu- 

 liarities of reproduction are readily understood when we recollect that 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 many species. 



Order I. Branchiopoda. 



The Branchiopoda are relatively large with numerous segments, leaf- 

 like appendages, long, chambered heart, and lack swimming antennae. 

 "With few exceptions they are inhabitants of 

 fresh water. According to the development of the 

 carapace they are subdivided into three families. 



1. APODID^E. Body depressed, with large oval 

 undivided carapace. Eggs carried in brood cap- 

 sules formed by a pair of appendages. Apus 

 (fig. 412) and Lepidurus occur in Europe, Green- 

 land, and the United States west of the Missouri. 

 Protocaris of the Cambrian rocks is apparently 

 an Apodid. 



2. BRANCHIPID^E. Body without carapace, the 

 second antennae of the male large and modified 

 into an organ for clasping the female. The female 

 carries the summer eggs in a wide ' uterus ' in 

 the abdomen. Branchipus lives in fresh water, 

 Artemia in brine, and in certain species one has 



FIG 41* 4 DM* equalis* been transformed into the other by changing the 

 ' (After^Packard.) water from fresh to salt or the reverse. Branchi- 

 pus vernalis (fig. 419) occurs in snow-water pools in our northern states, 

 Artemia in Great Salt Lake, around salt works or in tubs of brine near 

 the ocean. 



