278 ARANEIFORM CRUSTACEA; PYCNOGONIDA. 



Entomostracous Crustacea (Fig. 496), that it would be im- 

 possible to show any decided difference between them. This 

 is a very interesting example of the importance of becoming 

 acquainted with the whole Natural History of an animal, whose 

 place in the scale is at all doubtful. 



ORDER XII. ARANEIFORMES. 



816. This is a very remarkable group, which is only referred 

 to the position here assigned it, because it can scarcely be placed 

 in any other, at least without a more exact knowledge of its 

 internal structure and development. It consists of the small 

 family PYCNOGONIDA ; which, in regard to the general form of 

 the body, and the completeness of the extremities, seems to take 

 rank rather with the higher Crustacea, or with the Arachnida. 

 With the latter they have been commonly associated, on account 

 of the number of their legs ; but they differ in the absence of 

 breathing pores, by which the want of in- 

 ternal respiratory organs may be inferred ; 

 and in their marine residence. There is 

 reason to believe that the digestive cavity 

 is not confined to the narrow body ; but 

 that it sends prolongations into the legs, 

 as it does into the rays of the Star-fish ; 

 and from this circumstance, as well as from 

 the absence of a respiratory apparatus, the circulating system may 

 be regarded either as incomplete, or as altogether wanting, the 

 respiration being provided for, as in the lower tribes of Crustacea, 

 by the exposure of the fluids of the body to the surrounding 

 medium. Further, the residence of these animals would seem 

 to remove them from the Arachnida ; for they are all marine, 

 many being found amongst plants or stones on the sea-beach, 

 and others being dredged up from deep water. Their minuteness 

 renders it difficult to ascertain many points of their structure ; 

 and much is still to be learned respecting them. Various species 

 exist on our own coasts. Their motions are remarkably slow 

 and apparently difficult ; and it is obvious from this circumstance, 



