HARD WICKE ' S S CIENCE- G O S.SIP. 



79 



fertilisation with the antherozoids that have been 

 produced by the antheridia (corresponding functionally 

 with the pollen of flowering plants), germinates and 

 gives origin to a fern plant which produces spores, 

 each capable of giving origin to a similar cycle of 

 changes. In this case there are two so-called genera- 

 tions, the first commencing with the germination of 

 the spore and terminating with the production of the 

 fertilised oospore, at which period the prothallium 

 perishes, this is styled the sexual generation, because 

 the oospore — equivalent to the fertilised ovule in 

 flowering plants — is the direct result of fertilisation ; 

 the second generation commences with the germina- 

 tion of the oospore and ends with the production of 

 spores on the fern plant, this is the asexual generation, 

 because the spore is not the result of direct fertilisa- 

 tion, and when sown could not give origin to a fern 

 plant without previously producing the sexual pro- 

 thalloid form. In this instance we have clearly re- 

 presented only one generation, not two ; when once 

 growth has commenced with the spore it goes on un- 

 interruptedly until another spore is formed, the fer- 

 tilised oospore, which is said to terminate the first 

 generation, not possessing the power of remaining in 

 a state of dormant vitality, as is the case with the 

 seeds of flowering plants, and which marks the end 

 of the individual that gave origin to the seed, but 

 this property is possessed by the fern spore, therefore 

 one generation includes all the changes from the 

 germination of a spore until the production of another 

 similar one. The term "alternation of generations," 

 so far as concerns the vegetable kingdom, simply 

 expresses the fact that when active life has commenced, 

 a series of changes in form and function must be 

 passed through before the starting-point can be again 

 reached, or in other words before a body capable of 

 giving origin to a similar cycle can be repeated. In 

 fungi the "generations" are frequently several in 

 number, but they do not always follow in the same 

 order, the appearance of any one appears to be deter- 

 mined by surrounding causes, so that the plant 

 possesses the property of repeating itself under widely 

 different conditions. In ferns we have seen that the 

 sexual generation is microscopic and disappears — 

 except the oosphere — before the appearance of the 

 large asexual form or fern proper ; in mosses, on the 

 contrary, the sexual generation — the leafy part of the 

 plant — is largest and frequently perennial, giving origin 

 to several asexual generations — the capsules. The terms 

 prothallus or pro-embryo are vaguely defined, the 

 latter signifying everything produced anterior to the 

 embryo, consequently when a bulbil of Liliitm bulbi- 

 fertim developes into a plant the whole represents 

 a pro-embryo, as would also a potato plant origina- 

 ting from a tuber, both would also be examples 

 of asexual generations, whereas plants produced 

 from seeds of the above "would constitute the sexual 

 generation. 



G. E. Massee. 



A CHAPTER ON FISH PARASITES. 

 By John Davies, F.R.M.S. 



FISH parasites are a subdivision of the Entomo- 

 straca, and are divided into several species, 

 viz. : The Caligulus, having a sucking mouth and a 

 regular series of legs. They are sometimes called 

 "suctorial Crustacea." The Argulidre, which prin- 

 cipally infest fresh-water fish. The body is covered 

 with an oval shell, the abdomen is exposed. It has a 

 pair of sucking discs, or feet-jaws, and four pairs of 

 legs more or less articulated and generally plumose. 

 These parasites undergo a number of remarkable 

 changes and cast their shells at frequent intervals. If 

 a limb is lost it is replaced at the next moult, same as 

 crabs, lobsters, &c. These castings take place at 

 intervals of two or three days during some periods of 

 the year. The Argulidse are mostly found on fish in 

 a weakly state, or on those that have met with some 

 accident, which causes them to be more than usually 

 sluggish ; or on those that are by nature inert. The 

 carp offers a striking example of the latter class, and 

 the fact of its being more than usually infested has 

 given rise to a proverb. I do not think the fish suffer 

 in any way from the presence of these creatures — on the 

 contrary, if they feed on cutaneous secretions, it must 

 benefit their host, from a " hygienic point of view." 



These parasitic Crustacea are very quick in their 

 movements over their hosts, being able to travel back- 

 wards and forwards with equal facility. Their peculiar 

 mode of swimming has been described as a "series of 

 tumblings over and over, and darts in a straight line 

 with great rapidity." The fish seem to have a great 

 aversion to these messmates as an article of food, for 

 if by chance one gets down the throat of a fish it 

 immediately ejects it again, and would rather starve 

 than eat it. The female has generally two long 

 oviferous tubes for depositing her eggs (see Article 

 in Science-Gossip, page 33, vol. 1878). 



When the young animal comes forth it resembles 

 the Cyclops, and by successive moultings attains the 

 adult form. These metamorphoses do not apply to 

 the males, as they scarcely alter in form and only 

 slightly increase in size. 



It is a curious fact that most of these animals when 

 first hatched bear a great resemblance to the creatures 

 immediately below them in point of organisation. Their 

 cast-off shells, after being cleaned by the myriads of 

 minute scavengers (Monads) form most beautiful 

 objects for the microscope. They should be examined 

 with the half-inch objective in conjunction with the 

 spot lens, and as permanent objects can be preserved 

 in a solution of chloride of calcium, or glycerine-jelly. 



There is a great difficulty in examining these small 

 Crustacea as they soon perish after leaving their native 

 element, and in fact they seem bent on committing 

 self-destruction, as they generally climb out of the 

 vessel in which they are placed, and soon end their 

 existence. The Caligulus was first-mentioned by 



