222 Zoological Society. 



by Klein to London to Sir Hans Sloane, and at the very same time 

 (1738) this species was found also in England in a pond on Bexley 

 Heath by the Rev.Lyttleton Brown. Klein's notice, previously sent to 

 Sir Hans Sloane, and Mr. Brown's description, were published simul- 

 taneously in the ' Philosophical Transactions ' of that year. Several 

 species have since then been discovered, natives of various parts of 

 the globe and having a wide geographical range. They have been 

 found in diiferent parts of Europe, in North Africa, in North Ame- 

 rica, even as high as the borders of the Arctic Ocean, in the West 

 Indies, and in Australia. 



The Family Apodidce (belonging to the Order Phyllopoda) may be 

 thus characterized : — 



Pedes branehiales, paribus sexaginta. Antennce breves, styli- 

 formes, pari singula. Oculi duo, sessiles. Corpus numerose 

 articulatum, parte majore clypeo magna obtectum. 



The feet, consisting of 60 pairs, are all formed for the purpose of 

 breathing with, and not for locomotion, the first pair alone being pro- 

 vided, in addition to the branchial plates, with organs adapted for 

 assisting the animal in swimming. The first pair are the largest, 

 and after the second pair they become gradually smaller as they de- 

 scend, until the last few pairs become almost obsolete. The animals 

 generally swim on their back, and these branchial feet are in constant 

 motion even when the animal is at rest. The body is cylindrical, 

 elongated, consisting of mimerous segments, and the upper half, or 

 more in some species, is covered by a large shield-shaped carapace or 

 buckler. This carapace protects the vital parts, and is furnished with 

 a peculiar structure in its substance for increasing the extent of its 

 branchial apparatus. The antennae are small organs and in number only 

 one pair, short and styliform. The eyes are two in number, compound, 

 lunate-shaped, and are sessile, being placed on the upper and central 

 portion of the carapace. The young have only cue eye, which gra- 

 dually disappears as moulting goes on, until the mark merely remains. 

 This is generally described as a third eye, but according to Zaddach 

 the two compound eyes only are provided with optic nerve, pigment 

 and cornea. The caudal segment of the body gives oif two long and 

 very numerously articulated cylindrical setee or filaments which are 

 more or less provided with short hairs from each side. 



Only one genus of this family is recognized by M. Milne-Edwards 

 in his work on the Crustacea, though Dr. Leach had many years 

 ago established a second ; the character upon which that genus was 

 founded by Leach is not considered by M. Edwards as of generic 

 importance ; but having observed another character equally remark- 

 able, which occurs in all the known species of the group which that 

 genus represents, I consider Leach's gen vis ought to be adopted, and 

 I now propose giving the characters of the two genera at greater 

 length than they have yet been done. 



Genus Apus, Scopoli. 

 Clypeus coi'nea-cariaceus. Corpus malle, cylindricum. Seg- 

 mentum caudate lamina product a non instructum. Pedum 

 primi paris appendices, out rami, longissimi, flexibiles. 



