AT THE MICROSCOPE. 2S3 



of intermittent suction. The eggs are laid in a mass underneath 

 the floating leaves of some aquatic plants, notably Polygonum 

 amphibhun. To these leaves they are firmly fixed by an abundant 

 secretion, that hardens under water to the consistency of dense 

 horn, or even becomes almost stony. Incubation occupies but a 

 few days in hot weather. When hatched out, the young, provided 

 with three pairs of limbs, robust palpi, and a pair of large eyes, 

 swim rapidly about in search of some water-insects, to which to 

 affix themselves, which is done by inserting the powerful beak 

 shown in the example before us. The favoured (?) hosts are ISIepa 

 cinerea and Dyticus marginalis, which may be sometimes found 

 hearing numerous examples. Their appearance now becomes 

 entirely changed ; the skin is cast, whilst the body greatly increases 

 in size, becoming meanwhile pear-shaped. This condition lasts 

 for some time ; during it, rudimentary limbs form internally, which 

 gradually undergo segmentation, and the seat of the future genital 

 orifice is distinguishable. The soft Hmbs are now withdrawn from 

 their investment, a pair of eyes become visible ; and when growth 

 has sufficiently proceeded, another moult takes place, the old 

 slough is cast, and they swim freely about like their parents. 

 After shedding the skin yet once more, the sexual organs become 

 fully developed, and they are ready to reproduce their kind. And 

 thus the curious cycle of their life history is completed. The 

 time of writing (July) is exactly that for repeating these observa- 

 tions, and I have not found HydracJma difficult to keep in aquaria, 

 under suitable conditions, long enough for the purpose. 



Those who desire fuller information will find it in an early 

 volume of " Aiinales des Sciences NaturelleSy^ a contribution by M. 

 Duges, or (abstracted) in the later editions of Prof T. Rymer 

 Jones's " Outlines of the Animal Kingdom^ 



In ^'■Science Gossip'' for November, 1865, at p. 255, occur 

 figures by Dr. Lewis G. Mills, of the larval parasite, evidently 

 taken from specimens mounted, and somewhat distorted in the 

 process. 



In connection with this slide, I should like such members as 

 can, at the first opportunity, whilst it is still fresh before us, to 

 exhibit the different phases in the creature's existence : — 



I. — The Eggs of Hydrachna globula. 



2. — The Young, newly hatched. 



3. — The Larva, as first attacking its host (the so-called " Ach- 

 lysia''). 



4. — The Larva in its second stage— the ^^ Leptus-forniy 



5. — The young creature in its free condition, with six limbs 

 only ; and, lastly, 



6 and 7. — Mature individuals of both sexes. 



