36 



PROVEEDINdS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



"lS. — A K KENT 

 SI"., I'ALPUS. 



mantle-folds of Lamcllibranch mollusk.s. During the course of devel- 

 opment a membrane is separated from the embryo while yet inclosed 

 in the egg-shell; the egg in this condition is the deutovum. After the 

 egg-shell is broken the eml)ryo continues to develop within this 

 deutovum. The larva on hatching is six-legged, and 

 in some cases is ]iarasitic in the respiratory system of 

 n:>ollusks or attached to a(}uatic insects. Neuniann 

 has claimed that in one species of L'nnnrxhi the form 

 hatchinii" from the euo- i.s eit>"ht-leoH)ed, and therefore 

 a nymph, as in J^tiro/ifns. Kramer has arranged the 

 larvie in three series, which he considers is the basis 

 of classification. As a whole, the larva^ difler greatly 

 from the adults and many were described as difl'erent 

 creatures. Some were the basis of the genus Achlyski.. 

 The larva usualh' has very small mouth- parts; it fas- 

 tens to an insect by means of hooks at the tip of the short, stout palpi, 

 inserts its jaws and proceeds to feed. Gradually the l)ody becomes 

 swollen, the legs shrink, and the creature looks like an elliptical Qgg^. 

 The pupa is formed within this sac-like ])ody, 

 and from it in time issues the adult mite. 



Nearly all Hydraclmida? live in fresh water, 

 a few forms occur in brackish water, and sev- 

 eral are known from the littoral zone of the 

 sea. Tliey are sometimes parasitic, as already 

 noticed, but usually free, and feed on any 

 small animals they can catch, such as snudl 

 Crustacea, infusoria, and minute insect lar\ie. 

 The water mites are found throughout the 

 globe, but appear to ))e most numerous in tem- 

 perate regions. Many species inhabit i'a})id 

 streams and very cold water. About 6<» genera and 60(» species have 

 been described. The species are often widely distributed and found 

 amid very diti'ering surroundings. 



\'arious classitications have been 

 made, one of which arranges them in 

 14 families. However, it may l)e bet- 

 ter in this paper to use a less complex 

 system. Some writers have kept the 

 Hydrachnida' (juite distinct from all 

 other mites, but they are now generally 

 recognized to be closely related to the 

 Trombidiidtv. Nordenskiold believes 

 the family to have two separate origins, and therefore not natural. 



In the following table are found several conmion European genera 

 which are not vet recognized from this country, and doubtless other 



Fig. 5'J.— .\KKENUKrs sp. 



M.\LE FKOM BELOW. 



Fig. go.— L.\rva of an hvdkaciina at- 

 taghed to leg of an insect: ny.ml'li 



INSIDE. 



