ARTICULATA. 121 
The order is widely spread, members of it having been found at Spitz 
bergen, the coasts of Europe and America, and Australia. Their habits 
are sluggish, and some of the species live together in considerable numbers. 
Kroyer has found that they have three stages of transformation, the body 
being in the first roundish or oval, without an abdomeh, but with cheliform 
mandibles even in Pycnogonum, the adult of which is without them, a fact 
which indicates the little importance of these organs, and the lower condition 
of the forms which retain them. The third pair of feet, the segments of the 
body, and the abdomen, appear in an undeveloped state in the second stage; 
and in the third, the last pair of feet are acquired, the preceding feet have 
become more perfect, and the body has become longer and more like that 
of the adult. Another moult brings the body nearly to its final form, whilst 
the feet, which had diminished in length from the first to the last. pair, become 
of equal length. 
This order is not extensive, but it contains a number of genera comprised 
in two families. 
Fam.1. Pycnogonide, in which the feet are comparatively short, the 
body rather robust, and the cheliform organs wanting. Pyenogonum (pl. 
78, fig. 32) has been erroneously stated to infest whales. 
Fam. 2. Nymphonide Nymphon (pl. 78, fig. 33, inverted), in which the 
body and feet are very slender, and having a pair of cheliform mandibles. 
NV. pallida, Say, 1821 (Anaphia), Jour. Acad. Nat. Sci. vol. ii. p. 60, pl. 
5, fig. 7, was found on the coast of South Carolina. The expanse of its 
feet is one and a half, and its length one fourth of an inch. 
Orver 2. Monomerosomata (pl. 77, jigs. 46, 47, 64-71). This order 
contains the small and generally microscopic spider-like animals known as 
mites and ticks, and to which an aerial and tracheary respiration is usually 
attributed, including those which are aquatic. According to Dujardin, 
Gamasus and other genera with cheliform mandibles, have traches, whilst 
Acarus and Sarcoptes breathe through the skin. He asserts further, that in 
Trombidium, inspiration takes place by the latter mode, and expiration by 
the former; and that in the aquatic genera respiration takes place through 
spiracles scattered over the surface. The body is not divided by the sepa- 
ration of the abdomen as in the ordinary spiders, nor are various segments 
apparent as in Chelifer and Scorpio. This being the case, when the 
anterior portion appears to form a head, it is by the enlargement of the 
haustellum or parts of the mouth, the eyes being in nearly every case situated 
upon the anterior part of the cephalothorax. The labium or lower lip 
supports or incloses the organs of manducation; the palpi are usually free, 
of five articulations, and they present many varieties of form which are 
useful in classification. In some they are adapted for seizing their prey, in 
some for holding, and in others for drawing their food towards the mouth. 
The feet are usually composed of seven articulations, including the coxa 
(which is either attached or movable), so that they correspond with those 
of the Areneide. ‘The extremity has usually two claws, capable of being 
thrown back and received into a corresponding cavity. The supposed 
Acari, with six feet, for which genera have been proposed, are the immature 
325 
