APPENDIX, 393 
stalk (curving upwards in our drawing); by means of it the 
Barnacle crab grows on rocks, ships, etc. On the ventral side are 
’ six pairs of feet. very foot is forked and divided into two 
long, curved, or curled “ tendrils” furnished with bristles. 
Above and behind the last pair of feet projects the thin cylin- 
drical tail. 
Fig. H c represents a parasitic sack-crab (Sacculina purpurea) 
from the order of Root-crabs (Rhizocephala). These parasites, 
by adaptation to a parasitical life, have developed out of Barnacle 
crabs (Fig. Dc), much in the same way as the fish-lice (Cc), 
out of the freely swimming Oar-legged crabs (B c). However, 
the suppression, and the subsequent degeneration, of all of the 
organs, has gone much further in the present case than in most 
of the fish-lice. Out of the articulated crab, possessing legs, 
intestine, and eye, and which in an early stage as nauplius (# n, 
Plate VIII.), swam about freely, there has developed a formless, 
unsegmented sack, a red sausage, which now only contains 
sexual organs (eggs and sperm) and an intestinal rudiment. The 
legs and the eye have completely disappeared. At the posterior 
end is the opening of the genitals. From the mouth grows a 
thick bunch of numerous tree-shaped and branching root-like 
fibres. These spread themselves out (like the roots of a plant 
in the ground) in the soft hinder part of the body of the hermit- 
crab (Pagurus), upon which the root-crab lives as a parasite, and 
from which it draws its nourishment. Our drawing (# c), a 
copy of Fritz Miiller’s, is slightly enlarged, and shows the whole 
of the sausage-shaped sack-crab, with all its root-fibres, when 
drawn out of the body upon which it lives. 
Fig. Fc is a shrimp (Peneus Miilleri), from the order of ten-foot 
crabs (Decapoda), to which our river cray-fish, and its nearest 
relative, the lobster, and the short-tailed shore-crabs also belong. 
This order contains the largest and, gastronomically, the most im- 
portant crabs, and belongs, together with the mouth-legged and 
split-legged crabs, to the legion of the stalk-eyed mailed crabs 
(Podophthalma). The shrimp, as well as the river crab, has in 
front, on each side below the eye, two long feelers (the first 
