162 ARACHNIDES. 



dilation, or if they have, the circulation is not complete. The 

 tracheae are divided near their origin into various branches, 

 and do not, as in Insects, form two trunks which run parallel 

 to each other throughout the whole length of the body and 

 receive air from various points by means of numerous stigmata. 

 Here, but two, at most, are distinctly visible, and they are 

 situated near the base of the abdomen(l). The number of 

 simple eyes is at most but four. They constitute our second 

 and last order, or that of the Tracheari^e. 



ORDER I. 

 PULMONARIiE(2). 



We here find a well marked circulating system and pulmo- 

 nary sacs, always placed under the abdomen, announced ex- 

 ternally by transverse openings or fissures (stigmata), of which 

 there are sometimes eight, four on each side, and at others 

 four, or even two. The number of simple eyes is from six to 

 eight(3), while in the following order it never exceeds four, 

 and is most generally but two ; sometimes they are hardly per- 

 ceptible, or even annihilated. The organ of respiration is 

 formed of little laminae. The heart is a large vessel which 



(1) The Pycnogonides exhibit no stigmata, and seem, in this respect, to approach 

 the last of the Crustacea, such as Dichelestium, Cecrops, and other Siphonostomous 

 Entomostraca. Savigny thinks they have a closer affinity to the Lsemodipoda, 

 from which, however, they are greatly removed, by the organization of the 

 mouth as well as by their eyes audjfeet. We still believe, however, from the en- 

 semble of their characters, that tlggy rather belong to the class of Arachnides, and 

 that they approximate particularly to Phalangkim with javhich various authors 

 have arranged them. We also think that they may respireby the surface of their 

 skin. At all events, we must await the results of anatomical investigation, be- 

 fore we can decide. 



(2) Usogata, Fab. 



(3) The Tessarops of Kafin., according to him, has but four eyes; I presume, 

 however, that the lateral ones escaped his notice. See the subgenus Eresus. 



