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Fresh-water Entomostraca. 

 By D. J. Scourfield, F.R.M.S. Read April i$th, 1899. 



It is my desire to try and interest you for a short time this evening 

 in a group of very common animals to which I have devoted most 

 of my attention for some few years past. The animals referred to 

 are the minute Crustaceans of our fresh waters known as Entomostraca, 

 or more popularly as water-fleas. 



Although this is not an entomological subject in the modern sense 

 of the term, a paper on the Entomostraca would have been quite in 

 order in days gone by, even before an exclusively Entomological 

 Society. The very name Entomostraca, i.e. "shelled insects," sug- 

 gests at once that these animals were probably at one time regarded 

 as insects. So, indeed, they were, for we find that Linnaeus and many 

 later naturalists placed the Entomostraca, and in fact all the Crustacea, 

 in the Apterous order of the Insecta, and in many of the older 

 entomological journals papers will be found dealing with these 

 animals. 



The Entomostraca constitute the lowest of the chief subdivisions 

 of the class Crustacea, which, as you are well aware, form with the 

 insects, spiders, centipedes, etc., the great zoological group of the 

 Arthropoda. The Entomostraca, therefore, are the near allies of 

 such well-known animals as the crabs, shrimps, lobsters, etc. They 

 are, however, very much smaller. The largest forms are rarely much 

 over 1 inch long, and the vast majority are under £ inch ; the smallest 

 are under T ^ inch. 



[A detailed description of Daph?iia pulex, Cyclops serrulatus, and 

 Cypris fuscata, illustrated by diagrams, was given here to show the 

 differences between the three most important orders of the Ento- 

 mostraca, viz. the Cladocera, Copepoda, and Ostracoda. The 

 peculiarities of the two remaining orders of fresh-water Entomostraca, 

 viz. the Phyllopoda and the Branchiura, were also briefly alluded to.] 



In their development many of the Entomostraca exhibit two very 

 distinct stages. The young hatch as little ovoid unsegmented free- 

 swimming larvae, provided with only three pairs of appendages. 

 These larvae are known as Nauplii. After moulting several times they 

 suddenly change into forms having the essential characteristics of the 

 adults of the species, or at least of the genera, to which they belong. 

 The young of the Cladocera, however, hatch out very similar in 

 appearance to their parents, though there is one curious exception 

 to this, viz. that the young of the remarkable Leptodora hyalina 

 hatched from the winter eggs have the form of Nauplii. 



