PITYLLOPODA. 



301 



Mollusca But they are especially characterized by the 

 broad leaf-like feet, subdivided into lobes, and adapted for 

 breathing as well as 

 for swimming. The 

 thorax merges insens- 

 ibly into the abdomen. 

 The number of body- 

 segments varies great- 

 ly, there being six- 

 teen in Limnetis, the 

 simplest form, and 

 sixty -nine in Apus, 

 or three times the 

 number present in the 



lobster, the Segments Fig. ai3. Limnetis GmldU, much enlarged. After 



thus being irrelatively ] 



repeated, a sign of inferiority. There is a pair of simple 

 eyes consolidated into one as in Limnetis and Limnadia, or 



as in Apus, there is a 

 pair of compound eyes, 

 situated in the cara- 

 pace, apparently on 

 one of the antennal 

 segments. In Bran- 

 chipus and Artemia 

 the compound eyes 

 are stalked, an antic- 

 ipation of the stalked 

 eye of the lobster, 

 etc., but the eye, it 

 should be noticed, is not developed from a separate 

 segment, but from one of the two antennal segments. All 

 the members of this order hatch in the Nauplius form, the 

 three pairs of appendages of the larva, representing the two 

 pairs of antennae and the mandibles of the adult. The spe- 

 cies live in pools of fresh water liable to dry up in summer ; 

 they lay eggs which drop to the bottom, and show great vi- 

 tality, withstanding the heat and dryness after the water 

 has evaporated ; the young hatching after the rains refill the 

 pools or ditches. 



Fig. 244. Limnadia Agassizii, enlarged. 



