ARACHNIDS. 567 



Most arachnids feed on other animals, which they either swal- 

 ;low alive, or whose blood and fluids they suck. Usually, after 

 (their escape from the egg, they undergo no metamorphosis. They 

 least, however, their skin more than once, and are commonly, after 

 (the fourth or fifth moult, in a state for pairing. In most Acarina, 

 |the young animals are at first supplied with only three pairs of feet, 

 which gave occasion to some writers to adopt some six-footed 

 qenera, which were afterwards rejected as unfounded. The Pycno- 

 qonida and the genus Hydrachna present the most interesting 

 changes of form, of which the most remarkable particulars will be 

 noticed in the systematic arrangement of these animals. 



The power of reproduction in arachnids is commonly, as in the 

 preceding class, considered to be small 1 . In many, however, lost 

 feet can grow again. Thus G-EOFFROY once saw a Phalangium, in 

 which one foot was less than the remaining seven 2 , and which pro- 

 bably might have grown at a later period. At all events it is 

 established that, in spiders, lost feet are regenerated 3 . In animals, 

 .*vhose growth is limited, i.e. which do not grow after they are 

 capable of propagating, I think the reproductive power, in this full- 

 i^rown state, is small. In such a case are insects after their last 

 , netamorphosis (see above, p. 276), but by no means spiders and 

 Crustaceans. 



Very various is the form of the nervous system in the arachnids. 



I'n the greatest number there is a large ganglion in the thorax 



\ cephalothorax) , formed, as it seems, by the coalition of different 



t>ther ganglia, from which the nerves for the under-jaws and. palpi, 



,nd for the four pairs of feet, radiate. At the posterior margin of 



jhis ganglion arise, under acute angles, close to each other (as in 



I he cauda equina of mammals), the nervous trunks for the abdomen. 



Che two middlemost are sometimes thicker, lie closer together, and 



mite towards the extremity, before dividing, to form a ganglion 



1 MECKEL'S Syst. der vergl. Anat. i. s. 121. 



2 Hist, abregee des Ins. i. p. 629. 



, 3 See an observation of the celebrated BANKS, interesting also in other respects 

 >3corded by LEACH Trans, of the Linn. Soc. xi. 1815, pp. 393, 394; see also HEINE- 

 EN'S experiments and observations, Zool. Journal, IV. 1829, pp. 284, 294, and those 

 If LEPELETIEB and AUDOUIN in TODD'S Cyclopcedia, i. pp. 214, 215. Spiders must, 

 'towever, lose the entire foot as far as the coxa ; if it be broken off lower the spider 

 ies, unless it succeed in breaking off the stump that is left. The new foot (at first 

 ery short) makes its appearance at the next moult. 



