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. 

 Kröyer has found that they have three stages of transformation, the body 

 being in the first roundish or oval, without an abdomen, but with cheliforni 

 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. 



Fmn. 1. PycnogonidoB^ in which the feet are comparatively short, the 

 body rather robust, and the cheliform organs wanting. Pycnogonum {pi. 

 '78, fig. 32) has been erroneously stated to infest whales. 



Fam. 2. NympTionidoß Nym])lion {-pi. 7S,ßg. 33, inverted), in which the 

 body and feet are very slender, and having a pair of cheliform mandibles. 

 JV.^Mllida, Say, 1821 [AnapMa\ Jour. Acad. Nat. Sei. vol. ii. p. 60, pi. 

 5, fig. T, 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. 



Order 2. Monomekosomata {pi. 77, figs. 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 cheliforni mandibles, have trachea3, whilst 

 Acarus and Sarcoptes breathe through the skin. He asserts further, that in 

 Tromhidium., 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 Areneidae. The extremity has usually two claws, capable of being 

 thrown back and received into a corresponding cavity. The supposed 

 Acwri.) with six feet, for which genera have been proposed, are the immature 



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