in PLATODES— ASEXUAL REPRODUCTION 163 



Copulation is generally mutual, both the copulating individuals acting as male 

 and as female. Self-fertilisation, however, also seems to take place, e.g. in Cestoda, 

 and perhaps also in a few Trematoda and Turbellaria. 



Development.— Like the atrium genitale, which is only a pit-like depression of the 

 outer skin, so the male and female copulatory apparati arise, at least according to 

 investigations made in the Polydada, by folds from the exterior. The portion of the 

 female genital organs which arises from invagination apparently reaches to the egg 

 passage or ootype, so that not only the glands which open into the male genital 

 organs, but the shell glands also of the female genital organs must be considered as 

 modified dermal glands. 



XIII. Asexual Reppoduetion and its Origin — The Organisation 

 of the Cestoda. 



ilany Platodes, and especially the Turbellaria, show a marked capacity for 

 regeneration. The body can not only re-form parts torn off, but broken off pieces of 

 various sizes can become regenerated into new animals. Such a capacity of regenera- 

 tion is very widespread, chiefly among lower stationary animals. In the CcelenUrata 

 it is almost universal. The great advantage of this capacity for the preservation 

 of the individual and of the race is evident. For attached animals, or very long 

 or delicate soft-bodied animals, who are more exposed than others to mutilation 

 and injuries to the body from enemies, etc., it is of very great importance. We 

 can perhaps trace back to it the power of asexual reproduction by fission and gem- 

 mation which occurs in the Metazoa. "We speak of such a method of reproduction 

 when an animal form shows the peculiarity of falling into 2 or more pieces, appar- 

 ently spontaneously, i.e. from causes unknown to us, these pieces becoming regener- 

 ated into organisms similar to the common mother animal ; or when, from unknown 

 causes, a smaller or larger piece of the body regularly detaches itself, the body 

 thus reduced again replacing the lost portion, while the detached part becomes 

 regenerated into a complete animal. 



Thus e.g. Lumhriculus, one of the worms belonging to the Oligochceta, falls 

 spontaneously, or apparently spontaneously, into 2 or more pieces, each of which 

 can become regenerated into a whole animal. Certain marine star-fish throw off one 

 or more arms apparently spontaneously, which they soon replace by regeneration. 

 As if this were not enough, each detached arm can again be regenerated into a 

 complete star-fish. 



This conjectural origin of asexual reproduction is, however, almost always un- 

 recognisable ; because different parts of an individual develop into whole individuals 

 before they have fully separated, or an animal replaces a part by regeneration before 

 this part has completely detached itself. Temporary animal stocks thus arise. If 

 the parts do not detach themselves typical animal stocks arise, which by division 

 of labour between the portions which are being regenerated into whole individuals 

 (i.e. between the individuals which have arisen by gemmation), and by the develop- 

 ment of a form and organisation in each adapted for some special function, may 

 become polymorphous animal stocks. 



The reproduction and life-history of the Acraspede Medusa, e.g. of Aurelia, 

 is specially suitable for the elucidation of the above view. We know that from 

 the fertilised egg of this Medusa, under certain circumstances, another Medusa 

 may proceed, without any attached stage multiplying asexually. Generally, 

 however, the larva developed from the fertilised egg attaches itself and becomes 

 a coral-like animal, the Seyphula, and later develops into an attached young 



