218 



KNO^VLEDGE. 



[Septkmber 1, 1890. 



it is only by a scientific study of this character that her 

 compasses can be guarded against the influence of local 

 magnetic action. As the constituents of this action are to 

 a large extent variable, it behoves the mariner to be ever 

 on the watch for causes which may interfere vath the reli- 

 ability of his " directive index." With the use of iron for 

 shipbuilding, and the introduction of electric lighting on 

 board ships, the necessity for increased watchfulness is 

 considerably augmented. With regard to the external 

 disturbing forces little danger is to be anticipated from 

 magnetic rocks diu-ing the day ; but at night, shipmasters 

 who love to shave corners would do well to ponder on the 

 instances of the vertical attraction of rocks adduced above. 



HARMLESS PARASITES AND UNINVITED 

 COMPANIONS.-U. 



By Henry -J. Slack, F.G.S., F.E.M.S. 



AMONGST the microscopic animals that are apt to 

 swarm upon other creatures, perhaps the com- 

 monest are members of the vorlicella or bell polyp 

 family. They are frequently so numerous as to 

 surround their victims like a cloud, and must be a 

 gi'eat nuisance to them. In these cases the assailan{s must 

 be regarded as parasites, though not of the worst sort, as 

 they do not feed upon their landlords. 



When an intestinal worm makes its abode in the liver, 

 bram, or muscle of some mammal, the parasitism is of the 

 worst land, for it destroys li\-ing tissue. In a milder set of 

 instances parasitic mvaders live like .Jonah in the stomachs 

 of their hosts, and nourish themselves upon some of the 

 food the latter collect. This is robbing a neighbour's 

 pantry of part of its contents ; but if plenty is left for his 

 own consumption, no great harm is done. The Hohithiiiidn, 

 or sea slugs, that supply microscopists with beautiful objeets 

 in the shape of the calcareous wheels, anchors, and curiously 

 perforated plates with which their integuments are beset, 

 also furnish many instances of parasitism. ' One species 

 aflbrds board and lodging in its stomach to a small eel-like 

 fish (Don:fU<() ; while others that collect a very liberal 

 commissariat are visited as boarders by prawns and pea- 

 crabs. Beneden tells us that his friend Semper foimd some 

 of these creatures in the Philippines who were not bad 

 imitations of an hotel furnished with a talile-dhote. 



Sea anemones and jelly-fish frequently act as hotel 

 keepers, and Dr. CoUingwood found in the China seas a 

 monster of the former kind ten feet in diameter, sheltering 

 in its interior some lively little fish. This is the more 

 curious as these animals are well armed with poison threads, 

 or flexible darts, which they project with considerable pene- 

 trating power ; and Gosse mentions an instance of a little 

 fish that died in a few minutes m great agony through a 

 momentary contact of its lips with one of these weapons. 

 For full and very interesting particulars of these organs 

 the reader is referred to Gosse's Jhitiali Sea Anvmones ; but 

 those who ^"isit the seaside with microscopes may easilj' 

 see some of them by snipping off a little piece of an 

 anemone's tentacle, and viewing it under compression with 

 a high power. One fonn of these weapons is shown in the 

 sketch copied from Gosse (Fig. 1|. The vesicle he calls a 

 cnida, and the thread coiled in it an ectliorca. This the 

 animal was flinging out with lightning velocity and gi-eat 

 force. 



Whether an anemone that plays the host to little fishes 

 derives any benefit or pleasm-e from their company we do 

 not know, but if it found them objectionable lodgers it 



would turn them out. It is often puzzling or impossible to 

 comprehend the human motives that impel to extraordinary 

 actions, and still more so is it to imagine the tvli)/ of many 

 doings of the lower animals. Why, for example, does one 

 species of hermit-crab, that has taken the shell of some 

 gasteropod for its dwelling, encourage the cloak anemone 

 {Aihniisia jiallwta) to fix itself on the lip of its abode? 

 Lieut. -Col. Stuart-Wortley tells us that the crab offers its 

 companion a share of its food captures, and assists the 

 anemone to move with it when it finds its home too small 

 and takes possession of a larger shell. No species of 

 hermit crab except PtitjiintH I'rideaumi forms this cmious 

 connection, and the most probable explanation of the 

 partnership is that suggested by Dr. Landsborough, cited 

 by Gosse. He says, in all likelihood they in various ways 

 aid each other. The hermit has strong claws, and while 

 he is feasting on the prey he has caught, many spare 

 crumbs may fall to the share of his gentle-looking com- 

 panion. But soft and gentle-looking as the anemone may 

 be, she has a hundred hands, and woe be to the wandering 

 wight who comes within reach of one of them, for all the 

 other hands are instantly brought to its aid, and the hermit 

 may soon find that he is more than compensated for the 

 crumbs that fall from his own booty. 



Collectors of pond objects often find the fi-esh-water 

 polyp, and fi'equently discover it in association with an 

 infusorium, the Trichndina pedicuhis, which can attach 

 itseK to its host by a sucker, or run fi-eely over it by means 

 of cilia that do the work of legs. The currents it produces 

 by its ciliary organs must be useful to the polj-p, and it is 

 said to act as scavenger and to remove waste matter from 

 its friend's skin. Another 

 infusoria, AVro«aj)o/;/j/(i/H>H, 

 also settles on the polyp, and 

 Kent says that " where the 

 two abound, it may be not 

 unfrequently observed that 

 the Triclwdina movmt upon 

 the backs of their compa- 

 nions, and thus utilise them as a man might a horse, for the 

 enjoyment of locomotion, without having to participate in 

 the labour of its production." Like the anemone and the 

 jeUy-fish, the polyp has poison thi'eads, and could destroy 

 its visitors if it objected to their presence. 



A great many other instances might be given of animals 

 boarding together in fi'iendly harmony or actual co-opera- 

 tion. Amongst the crustaceans such partnerships are 

 numerous, and a tiny pea-crab that makes its abode in 

 mussel-shells is evidently welcome to its shelter. 



The ancients, Beneden says, noticing the mussel's blind- 

 ness, thought the crab's sharp eyes enabled it to see by 

 deputy ; but a more probable solution of the question is 

 that the crab is a good food-catcher, and the mussel enjoys 

 part of the spoils. 



Fishes are visited by numerous animal and vegetable 

 colonists, some hannless and others destructive. In all 

 quarters we find both fellow-boarders and parasites, and it 

 is to be remarked that while the latter have always 

 attracted attention, the former, with few exceptions, have 

 only recently been studied as they deserve. 



Fig. 1, after Gosse, represents a very elaborate but 

 "most generally distributed form of Chamhered Cnidff"; 

 the vesicles are about -004 inch long, and in greatest 

 diameter -0005 inch. Inside the vesicle is a slender, 

 lozenge-shaped chamber, from which proceeds a very long 

 convoluted tube, often from twenty to forty times the length 

 of the Cnida. Gosse describes the protrusion of the 

 poison -thread as beginning by a nipple-shaped wart 6'om 

 the anterior extremity, and often proceeding slowly till it 



Fig. 1. — Chambered Cnid.e. 



