188 Proceedings of the Royal Irish Academi/. 



Their living period is limited to a couple of months and sometimes even less. 

 The perpetuation of the species is secured by the production of eggs, which, 

 when discharged from the carapace, drop to the bottom of the pool, remaining 

 in the mud when the poul dries up. Sars has shown by experiment that 

 these eggs will not hatch out until they are dried thoroughly, perhaps during 

 several successive years or drouglit periods. Each egg is provided with 

 curious wing-like expansions, not unlike the "wings "of many of our tree- 

 seeds, and in all probability these enable the eggs to be dispersed liy wind. 

 The eggs are produced in great numbers, have tremendous vitality and by 

 reason of their tenacious germinating power may be compared ro the seeds 

 of many plants. They seive lo carry the species over one or more probably 

 several long droughts, and hatch out into simple Nauplins larvae when the 

 hollows are once more filled with pools of water. 



The larval development only occupies a few days, during which time great 

 changes tjxke place and tlie rudiments of the carapace appear. This stage is 

 followed by a post-larval stage, in the course of which the carapace develops 

 as a bivalve shell, the lines of growth, as in the case of most Concliostracan 

 shells, representing successive moiilts, and finally, after about one month. 

 the adult animal is perfected. It lives in a sexually mature form rarely more 

 than a month, during which lime it provides for the continuance of the 

 species by laying, in successive batches, many thousands of eggs. Kaf h ImIi h 

 of eggs is laid in the interval between two moult periods. 



Thus these animals fhow a inarkeil degree of adaptation to their environ- 

 ment The pools in which they hatch out may never be pools again, and 

 so their brief life is spent in producing eggs, specially adapted for wide 

 distribution and great drying, so that in whatever hollow the rain may 

 accumulate in tiie next season it will provide a suitable hatching pool for 

 the eggs brought there by the wind. 



The order Conchostraca is divided into two families— the Limiiadiidae, 

 which includes the seven genera — Liniiiadia (Brongniait), Eiiliiiinadia 

 (Packard), Liiniiadella ( Packard), Estheria (Iliippell , Cyclestheria(G. O. Sars , 

 Leptestheria (G. O. Sars), and Limuadopsis (Spencer and Hall) ; and the 

 Limnetidae, which includes the single genus Limnetis (Loven). 



Very little is known as regards the fossil representatives of the Con- 

 chostraca. P.ivalve shells referable to the order appear in the Old Eed 

 Sandstone, and are found in fresh-water deposits of all the formations 

 from that era up to the present day. In the early days these shells were 

 considered to be inollu.scan, and were referred to such genera as Posidononiya 

 and Cyclas. In 1862, however, T. Kupert Jones' demonstrated that their 



' T. Rupert Jokes : A Monograph of the Fossil Estheriae. Pal. Soc., 18C2. 



