ARACHNIDS. x 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 
cast, 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 
genera, which were afterwards rejected as unfounded. The Pycno- 
gonida and the genus LHydrachna 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'. In many, however, lost 
feet can grow again. ‘Thus GEOFFROY once saw a Phalangium, in 
which one foot was less than the remaining seven?, and which pro- 
bably might have grown at a later period. At all events it is 
established that, in spiders, lost feet are regenerated’. In animals, 
whose growth is limited, ¢.e. which do not grow after they are 
capable of propagating, I think the reproductive power, in this full- 
grown state, is small. In such a case are insects after their last 
metamorphosis (see above, p. 276), but by no means spiders and 
crustaceans. 
Very various is the form of the nervous system in the arachnids. 
In the greatest number there is a large ganglion in the thorax 
(cephalothorax), formed, as it seems, by the coalition of different 
other ganglia, from which the nerves for the under-jaws and palpi, 
and for the four pairs of feet, radiate. At the posterior margin of 
this ganglion arise, under acute angles, close to each other (as in 
the cauda equina of mammals), the nervous trunks for the abdomen. 
The two middlemost are sometimes thicker, lie closer together, and 
unite towards the extremity, before dividing, to form a ganglion 
1 MECKEL’S Syst. der vergl. Anat. 1. 8. 121. 
2 Hist. abrégée des Ins. 1. p. 629. 
3 See an observation of the celebrated Banks, interesting also in other respects 
recorded by LEeacnH Trans. of the Linn. Soc. Xt. 1815, pp. 393, 3943; see also HEINE- 
KEN’S experiments and observations, Zool. Journal, Iv. 1829, pp. 284, 294, and those 
of LEPELETIER and AUDOUIN in Topp’s Cyclopedia, 1. pp. 214, 215. Spiders must, 
however, lose the entire foot as far as the cova; if it be broken off lower the spider 
dies, unless it succeed in breaking off the stump that is left. The new foot (at first 
very short) makes its appearance at the next moult. 
