SECT. vu. CR USTA CEA. ^ \ \ 



Pycnogono'idea or Spider Crabs. 



Some of the Spider crabs hook themselves to fishes, 

 while others live under stones, or sprawl with their long 

 hairy legs over sea- weeds, and feed on the gelatinous 

 matter these weeds afford. The throat with its mem- 

 bers, and the head soldered to its first ring, forms 

 nearly the whole animal. It has a pair of antennae and 

 four rudimentary eyes, set on a tubercule. A proboscis- 

 like projection extends from the front ; the mouth is fur- 

 nished with cilia and one pair of foot-jaws. Four pairs 

 of long hairy legs proceed from the throat, spread widely 

 on each side, and end in a hooked claw. The stomach, 

 which occupies the centre of the animal, sends off five 

 pairs of long closed tubes like rays ; one pair enters the 

 foot-jaws, the others penetrate the legs. This digesting 

 system is in a state of perpetual vermicular motion, 

 which, as well as the movements of the animal itself, 

 aerate its transparent blood through the skin, by keeping 

 it in circulation. So this insignificant-looking creature 

 has a very curious and complicated mechanism. 3 



Fossil Crustacea. 



Analogues to the Anomura are found in the Chalk 

 formation, but the Macrura are the prevailing forms. 

 Extinct species of lobster, crawfish, and shrimps are 

 met with in the secondary strata, from the Chalk to the 

 Coal measures. In the Coal formation all these higher 

 forms disappear, but then the gigantic King Crab, or 

 Limulus, is found accompanied by the minute Entomo- 

 stracan forms in infinite variety of species. 



8 ' Histoire naturelle des Crustacea,' par M. Milne-Edwards. 



p 2 



