392 APPENDIX. 



Fig. C cis a parasitic Oar-legged crab (Lemaeocera 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 gdls 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 dai-k 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 Lern^ocera is 

 represented in half profile, and is copied from Glaus. (Compare 

 Glaus, " Die Copepoden-Fauna von Nizza. Ein Beitrag zur 

 Gharacteristik der Formen und deren Abanderungen im Sinne 

 Darwins." Marburg, 1866). 



Fig. D c represents a so-called "duck mussel" (Lepas 

 anatifera), from the order of the Barnacle crabs (Girripedia). 

 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 moUusc. It was only from a 

 knowledge of their ontogeny, and their early nauplius form (D n, 

 Plate VIII.), 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 



