130 



THE HELMINTHES. 



^119. 



§ 119. 



With the Nematodes, of which very many are viviparous, the embryos 

 are developed within the egg in two diiferent ways : Either the embi'yo- 



movements (Bner, in the Nov. Act. Acad. Lenp. 

 XIII. pt. 2, p. 570, Tab. XXX.). Those of Dis- 

 tomum duptic.atum liave simple, oval, and rigid 

 germinative utricles (Baer, Ibid. p. 558, Tab. 

 XXIX.). Those of Cercaria ephemera, are also 

 very simple, but of a cylindrical form (Siebold, ia 

 Burdach's Ph.ys. loc. cit. p. 187, and Steenstriip, 

 loc. cit. p. 78, Taf. III. fig. 1-6). Those of Cer- 

 f.aria furcata are simple and cylindrical, but very 

 ilong and endowed with quite active peristaltic 

 motions {Baer, loc. cit. p. 626, Tab. XXXI. iig. 6). 

 The curious animal, Leucocliloridium paradox- 

 U7n, consisting of only a cylindi-ical sac with a 

 tail, is only a trematode larva {Cams, in the Nov. 

 Act. Acad. Leop. XVII. pt. 1, p. 85, Tab. VII.). 

 \\ ith the slow-moving, cylindrical, orange-colored 

 nurses of Cercaria ephernera, there may be easily 

 seen a mouth, a pharynx, and a simple ooecal 

 intestine {Siel/old, in Burdach^s Phys. loc. cit. p. 

 187). Those of Cercaria echinata, are similar, 

 but they have also two short oblique prolongations 

 in front of the obtuse caudal extremity {Baer, loc. 

 cit. p. 629, Tab. 31, fig. 7, and Steenstrup, loc. cit., 

 p. 51, Taf. II. fig. 2-4). The germinative bodies 

 from which Cercaria is developed, have nothing 

 comparable to a chorion or germinative vesicle. 

 Their larvae have always a tail, which is simple 

 (^Cercaria armata, ephemera, Distomum dupli- 

 catum), or bifurcated {Cercaria furcatu), or 

 double {Bucephalus polymorphus). The move- 

 ments of this tail are very slow with Distomum 

 duplicatum, but extremely lively and vortical 

 with Cercaria. With Bucephalus, the two fiU- 

 form tails lengthen and shorten considerably, at the 

 same time jerking all about. 



When the larvae are developed, they leave the 

 corpuscles and pass into other animals to complete 

 their final metamorphoses. Many Cercariae appear 

 to prefer the larvae of insects whose bodies they en- 

 ter by means of their cephalic hooks. In this way 

 I have seen the Cercaria armata easily enter the 

 larvae of Ephemera, Nemura, and Perla. By the 

 aid of its sting it can perceive the inttrsegmental 

 membrane of these larvae. Frequently it loses its 

 tail in passing through a narrow opening it has 

 made. 



Immediately upon reaching the cavity of the 

 body of the larva, it is surrounded by a vesicular 

 membrane, in which the sting is rejected, and the 

 animal enters upon its final metamorphosis. But 

 I have a doubt whether it is there cumpleted, for 

 among the numerous similar parasites which I have 

 found in the most different insects whose larvae are 



aquatic, as oi Libellula. Adrian, Ephemera, and 

 Phryganea, I liave never met with one whose 

 genital organs were in a state of advanced develop- 

 ment. 



The full development of these organs, the deli- 

 cate contours of whicli may be seen while the par- 

 asites are in the bodies of these animals, is not 

 perhaps attained, until the insects have been 

 swallowed by birds and other animals, — being 

 thereby furnished with more proper conditions for 

 their complete formation. 



Some Cercarjae lose their tail and are surrounded 

 with a capsule without leaving the Mollusks which 

 are their first habitat. This is prolrably so, be- 

 cause these Mollusks are liable to be eaten by 

 aquatic birds, in whicli these parasites may prop- 

 erly reach their final development. It should, 

 however, be remarked that when these larvae be- 

 come chrysalides, their investing capsule or cyst, 

 is a secretion from their bodies, and not a product 

 of the animals in which they live. It is iirobable 

 that very many of these larvae never attain a per- 

 fect state, for, in their migrations, they fail to reach 

 their destined and final habitat. 



These migrations undoulrtedly occur with many 

 Cestodes while young ; at least Miescher (loc. cit.) 

 has observed it with Tetrarhynchus. But although 

 we have followed these in their migrations, and the 

 transformation of many of them into Monostomum. 

 and Distomum has been observed, and therefore 

 the completion of their metamorphoses, yet we are 

 but slightly informed as to their beginning by the 

 alternation of generation. 



There is yet little known as to the manner in 

 which these embryos are changed hito the cylin- 

 dric nurses. There are now only two isolated facts 

 throwing light upon this point. According to my 

 own observations {IViegmami's Arch. 1835, I. 

 p. 75, Taf. I.), each embryo of Monostomum 

 mutatiile contains a germinative tube, which, at 

 the death of the embryo, is freed and quite resem- 

 bles the nurse of Cercaria echinata. I have also 

 observed in the embryos of Amphistomum sub- 

 clavatuvi a tubular body, but I could not satisfy 

 myself of its germinative nature. According to 

 Steenstrup (loc. cit. p. 98), there is an animal like 

 a Paramaecium, and probably an embryo of a 

 Distomum, living in Muscles, and which finally 

 is deprived of its epitljclium, an<i changed into the 

 rigid, germinative tu'n' nf Difiliimum. duplicatum ; 

 see upon this, my .lahresbericht in fVieg?nann's 

 Arch. 1843, II. p. 30Ü.* 



* [^ 118, note v.] In this connection should be 

 noticed the remarkable phenomena of reproduction 

 with Gyrodactylus as recently observed by Sie- 

 bold {Siebold and Külliker's Zeitsch. I. 1849, p. 

 345). Individuals are here developed vivijiarously 

 as in the so-called alternating generations, and 

 Siebold has observed a mother in which was a 

 daughter and in this last a grand-daughter, the 

 series being therefore three-fold. These viviparous 

 individuals contain no sexual organs proper, but 

 the new individual is developed out of a group of 

 cells situated within the body. The whole repro- 

 ductive conditions which Siebold has detailed with 

 his usual care api)ear to me to closely resemble 

 those of the viviparous Aphides which I have 



recently investigated ; and I believe this mode of 

 reproduction to be only a peculiar form of gemmi- 

 parity or budding suited to some ulterior, econom- 

 ical purpose of the animal's life. On a future page 

 I shall speak more fully on this point and attempt 

 to show that the whole set of phenomena known 

 under the name of " Alternation of Generations " 

 is, when divested of its parai)hernaUa, only a kind 

 of Gemmiparity. 



See also for further details on that curious ani- 

 mal Leucochloridium paradoxum. Piper, in 

 fViegmann's Arch. 1851, I. p. 313, but especially 

 Siebold, in Siebold and KültikerKt Zeitsch. IV. 

 1853, p. 425, Taf. XVI. B. This last-named ob- 

 server has shown that tliis animal form is only a 



