286 ARANEIFORM CRUSTACEA; P7CNOGONIDA. 



eleven segments, one forming the head, seven belonging to the 

 thoracic region, and three to the ahdomen, and all destitute of 

 feet ; the only external organs being a pair of threads, which 

 Mr. Darwin regards as the representatives of the carapace, and 

 which enclose the cement-ducts ( 893), and bear the three- 

 jointed antennae. This curious species is parasitic in the sac of 

 the Alepas cornuta, a West Indian species of Barnacle, where it 

 is firmly cemented down by its antennas ; its mouth is suctorial, 

 and it no doubt lives upon the juices of its host. 



SECTION F. 

 ORDER XIV. AEANEIFORMIA. 



899. This is a very remarkable group, which cannot be re- 

 ferred definitely to any position until we possess a more exact 

 knowledge of its internal structure and development. It con- 

 sists of the small family PYCNOGONID^: ; 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 as- 

 sociated, on account of the number of their legs ; but they dif- 

 fer in the absence of breathing pores, by 

 which the want of internal respiratory or- 

 gans may be inferred ; and in their marine 

 residence. The digestive cavity is not con- 

 fined to the narrow body ; but sends pro- 

 longations into the legs, as it does into the 

 rays of the Star-fish ; and from this circum- 



stance > as wel1 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 sur- 

 rounding medium. Krohn, however, has lately stated that these 

 animals possess a dorsal vessel, similar in structure to that of the 

 Insects and Arachnida ; and they are said to possess traces of a 



