SEXUAL MATURITY. 43 



in locomotion and capture of its food, is small, generally less than in 

 free-living animals, and the income, therefore, is large ; there are in 

 fact, without going into any further detail, numerous causes which 

 must be considered as having a most important effect in furthering 

 sexual maturity. The large balance on the side of income explains 

 the great fertility, upon which stress has already been laid, as of 

 extreme importance in the life-history of these animals.^ 



This, however, is merely en passant. Most important is tJie fact 

 that sexual maturity and generation take place in most parasites 

 during the time of their parasitic life. Copulation is often accom- 

 plished in the lower animals before the female is fully developed, and 

 occasionally before the stage of parasitism commences. This is the 

 case, at least, in the Lernwo}, Avhere coition takes place while the ani- 

 mals are swimming freely in the water, ^ and differ but little from 

 the free-living Copepoda, and also in the chigoe {Pulex or Bhyn- 

 choprion penetrans, Fig. 28)- — it being supposed, at least, that onh- 

 stationary parasitism is to be taken into consideration. It is, moreover, 

 as is well known, only the female that is a stationary parasite. 

 While the male retains the ordinary form and habits of a flea, the 

 female bores her way into the skin of the foot in man, dogs, and other 

 mammals, and becomes, by the enormous development of the ovary, 

 a simple, motionless bladder. It is improbable, however, that there 

 is anything analogous to this in the Helminths. It was thought at 

 one time (Carter), but wrongly, that the Guinea-worm was fertil- 

 ised before it became parasitic ; but, as a matter of fact, this Nema- 

 tode is only found leading an independent existence in its earliest 

 stages, when the sexual organs are totally undeveloped.^ It is 



' We may give this instance of remarkable fruitfulness, in addition to that of the 

 Nematodes, to which allusion has already been made (p. 32). In Tania solium, the con- 

 tents of the uterus of each of the segments is about 6 cubic millimetres, and it holds some 

 53,000 eggs, each egg having a diameter of 0'06 mm. ; seeing that a tape-worm produces 

 yearly at least 800 segments, the total number of eggs will be thus some 42,000,000, a 

 number that under favourable circumstances — (instances are known of tape-worms budding 

 off five or six fresh segments daily) — is even occasionally exceeded. The extent of this 

 fertility may be estimated by the following calculations : — The 64,000,000 eggs, which, 

 according to Eschricht, a tape-worm brings forth in the course of a year, represent (each 

 egg being '05 mm. in diameter, and having a specific gravity equal to that of water), a mass 

 of 41,856 mgrm. (1 egg = '0000654 mgrm.). The adult worm itself weighs about 2'4 grm. or 

 3'4grm., including the ovarian tube, and produces therefore yearly 174 gr. per cent, of eggs, 

 about thirteen times as much as the queen bee, whose fertility is about 13 gr. per cent. 

 A woman in giving birth to a child is deprived of about 7 per cent, of her weight, so that 

 a thread-worm is as fertile as a woman would be if she brought forth seventy children 

 every day ! 



" Claus, " Beobachtungen iiber Lernseocera, Penioulus, und Lemaea," Scliriften dcr 

 Qesdlsch. zur Seforderung d. ges. Naturw. zu Marburg, Suppl.-Heft ii., p. 21, 1868. 



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