392 APPENDIX. 
Fig. C cis a parasitic Oar-legged crab (Lernzocera esocina), 
from the order of fish lice (Siphonostoma). These peculiar 
crabs, which were formerly regarded as worms, have originated, 
by adaptation to a parasitical life, out of freely swimming, Oar- 
legged crabs (Eucopepoda), and belong to the same legion 
(Copepoda, vol. ii. p. 176). By adhering to the gills on the skin of 
fish or other crabs, and feeding on the juice of these creatures, 
they forfeited their eyes, legs, and other organs, and developed 
into formless, inarticulated sacks, which, on a mere external 
examination, we should never suppose to be animals. On the 
ventral side only there exist, in the shape of short, pointed 
bristles, the last remains of legs which have now almost entirely 
disappeared. Two of these rudimentary pairs of legs (the third 
and fourth) are seen in our drawing on the right. Above, on 
the head, we see thick, shapeless appendages, the lower ones of 
which are split. In the centre of the body is seen the intestinal 
canal, which is surrounded by a dark covering of fat. At 
its posterior end is the ovary, and the cement-glands of the 
female sexual apparatus. The two large egg-sacks hang ex- 
ternally (as in the Cyclops, Fig. B). Our Lerneocera is 
represented in half profile, and is copied from Claus. (Compare 
Claus, “ Die Copepoden-Fauna von Nizza. lin Beitrag zur 
Characteristik der Formen und deren Abanderungen im Sinne 
Darwins.” Marburg, 1866). 
Fig. D ¢ represents a so-called “duck mussel” (Lepas 
anatifera), from the order of the Barnacle crabs (Cirripedia)- 
These crabs, upon which Darwin has written a very careful 
monograph, are, like mussels, enclosed in a bivalved, calcareous 
case, and hence were formerly (even by Cuvier) universally 
regarded as a kind of mussel, or mollusc. It was only from a 
knowledge of their ontogeny, and their early nauplius form (Dn, 
Plate VIITI.), that their crustacean nature was proved. Our 
drawing shows a ‘“‘duck mussel”’ of the natural size, from the right 
side. The right half of the bivalved shell has been removed, so 
that the body is seen lying in the left half of the shell. From 
the rudimentary head of the Lepas there issues a long, fleshy 
