48 THE INFLUENCE OF INANIMATE SUKKOUNDINGS. 



to zoologists as a Nauplius. This animal has a nervoHs systeiii, 

 external organs of locomotion of a complicated character, a mus- 

 cular system of the crustacean type, a well-developed: intestinal 

 canal such as is found in the Nauplius larvw of the lower crahs 

 that are not parasites, and usually even special organs of sense — 

 eyes. Gradually this Nauplius, after attaching itself to the 

 gill or skin of a fish, or under the tail of a crab (Sacculina), 

 loses its organs of locomotion, the greater part of its muscular 

 and nervous system, its organs of sense, nay, often its mouth, 

 stomach, and intestinal canal. Thus the lively crab-like larva 

 is transformed into a shapeless sac, exhibiting no trace by which 

 its crab-like nature can be recognised. Still the creature needs 

 a limb by which to cling to the animal that is to be its host 

 and provide it with nourishment ; peculiar clinging organs are 

 developed instead of the lost motory organs (fig. 1.2, a), and 

 these not unfrequently also assume the office of absorbing 

 nutrition from the host. Such, for instance, is the case with 

 the parasitical crabs, which, like Sacculina (fig. 12), live on the 

 abdomen of the hermit crab (Pagurus) or of other crabs. 

 They have, without exception, long filamentary processes at the 

 fore-end of the body, with which they cling and bore through 

 the skin of the crab into its abdominal cavity, and then they 

 clasp portions of the crab's internal organs, particularly the 

 liver, in the long entangled filaments. These slender threads 

 are thin-coated tubes which open into the body cavity of the 

 parasite, so that it is highly probable that these cliuging threads 

 also serve the purpose of suckers, since they are capable of- 

 absorbing nourishment in a fluid form through the:r thin tissue ; 

 at any rate, they do not convey it into an intestinal canal, for 

 the parasite has none. This, however, is no argument against 

 the assumption that the fluid thus absorbed by endosmosis 

 through these roots or suckers serves as food ; for we know that 

 in all animals which have a body cavity and dispense with a 

 vascular system the food must first pass into the body cavity, 

 in order to be conveyed from thence to the organs situated on 

 it. So far as regards the, part taken by the clinging filaments 

 as organs of nutrition to the parasite, it is perfectly indifferent 

 whether the nutritious fiuids to be assimilated first pass through. 



