244 ARTHROPODS. 



Limnadia, with bivalve shell. Periodically partheno- 

 genetic. A mollusc-like bivalve shell is still more 

 marked in Estheria. 

 Apus, a fresh-vifater form with a large dorsal shield. 

 Periodically parthenogenetic. 

 Of these Apus is certainly the most interesting. . It is over an inch in 

 length, and therefore a giant among Entomostraca. It has an 

 almost world-wide distribution. " It possesses peculiarities of 

 organisation which mark it out as an archaic form, probably 

 standing nearer to the extinct ancestors of the Crustacea than 

 almost any other living member of the group. " From E. Ray 

 Lankester's description of yy/2a (Quart. Journ. Micr. Sci., xxi., 

 1881), I take the following list of the appendages : — 



!i. Antenna. 

 2. Second antenna. (Like many others, I have been 

 unable to find this. It is sometimes absent, and 

 apparently always in certain species). 

 !3. Mandible. 

 4. Maxilla. 

 5. Maxillipede. 



i6. First thoracic foot (leg-like). 

 7-16. Other ten thoracic feet (swimmers). 

 The l6th in the female carries an egg-sac or brood- 

 chamber. There are eleven thoracic rings on the body. 

 Abdominal I 17-68. Fifty-two abdominal feet, to which there corres- 

 ( Post-genital). | pond only 17 rings on the body. 



The large dorsal shield is not attached to the segments behind the one 

 bearing the maxillipedes. Many of the thin limbs doubtless func 

 tion as gills. The genital apertures are on the 1 6th appendages. 

 The anus is on the last segment of the body. 

 The ventral nerve-cords are widely apart ; and the cephalic ganglion is 

 remarkably isolated. Lankester called the cephalic or supra-oeso- 

 phageal ganglion an " archi-cerebrum," to emphasise its preoral 

 position and its distinctness from posterior ganglia. Subsequent 

 research has shown, however, that in Apus as in other Crustaceans, 

 the cephalic ganglion is a "syn-cerebrum," that is to say, com- 

 bined with the post-oral ganglia which have been shunted forwards. 

 (b) Cladocera. Small laterally compressed Crustaceans, with few 

 and somewhat indistinct segments. The shell is usually 

 bivalved. The head often projects freely. The second pair 

 of antennae are large, two-branched, swimming appendages, 

 and thereare4-6 pairs of other swiniming organs. The heart is 

 a little sac withonepairof openings. Thefemaleshaveabrood- 

 chamber between the shell and the back. Wit in this many 

 broods are hatched throughout the summer. Periodic par- 

 thenogenesis (of the "summer-ova") is very common. "Win- 

 ter-eggs," which require fertilisation, are set adrift in a part of 

 the shell modified to form a cradle or ephippium. 

 Daphnia, Moina, Sida, Polyphemus, Leptodora, and many other 

 " water-fleas " are extraordinarily abundant in fresh water,, 

 and form a part of the food of fishes. 



