SEXUAL PARASITES OF INVERTEBRATES. 117 



Archigetes (Fig. 73) is, morphologically speaking, nothing else than a 

 Cysticercoid a tape-worm which concludes its metamorphosis at a 

 stage of development which, in the case of the common Cestodes, 

 represents merely a transitional form inhabiting an intermediate host. 

 Aspidogaster also (Fig. 74) is wrongly classed among the otherwise 

 ectoparasitic Polystornidse, on account of an absence of metamor- 

 phosis, whilst its structure stamps it decidedly as a Trematode allied 

 to Distonmm. Aspidogaster resembles a Redia in its mode of de- 

 velopment and the formation of its intestinal apparatus in so 

 remarkable a manner, that I see no objection to placing it, notwith- 

 standing its sexual maturity, beside the true Distomidse, and so classify- 

 ing it along with them, just as Archigetes is placed with the tape-worms. 

 The presence of a ventral sucker can as little be opposed to this 

 conception as the high development of the excretory system of vessels, 

 since both structures must be regarded merely as the result of an 

 adaptation to the animal's mode of life, which cannot be taken into 

 consideration in determining morphological relationships. 



The Redise and the Sporocysts (Fig. 49), which have sprung 

 from them by a retrograde formation of the intestine, 1 are, in 

 accordance with the above discussion, to be regarded as the oldest 

 Distomidse, in the same way as the Cysticercoids are the original 

 tape-worms. This agrees with the fact that the Redise are more closely 

 related to the ectoparasitic Trematodes (specially by the structure 

 of the intestine) than are the fully formed Distomidse, and hence 

 may be more easily and readily supposed to be derived from the 

 former. It is, moreover, sufficiently known that the Redise do not 

 change directly into the mature Distomidse, but develop them in their 

 body-cavity out of so-called " germ " cells, which are of the nature of 

 eggs, and separate themselves from the body-wall (Fig. 75). The 

 metamorphosis is divided over two generations, which spring from 

 each other; it thus becomes an alternation of generations, a common 

 phenomenon, as has been shown above. The production of the new 

 brood may perhaps in this case be directly connected with the former 



1 [This supposition has found an unexpected confirmation in the discovery of the 

 Orthonectida (see Giard, Jaurn. de I'Anat. et Phys., t. xv., p. 449, 1879, and Metschnikoff , 

 Zcitschr. /. wiss. Zool., Bd. xxxv., p. 282, 1881) ; or rather through the establishment of 

 the fact that these simple animals, parasitic on Ophiuroids and Turbellarians, are to be 

 regarded morphologically as sexually mature Trematode -embryos, devoid of an alimentary 

 canal (Leuckart, Archiv /. Naturgesch., Jahrg. xlviii., p. 96, 1879). Hence the Ortho- 

 nectida stand at the lowest stage of that series of developmental stages represented by 

 the Trematoda. Aspidogaster, therefore, which we have regarded as a sexually mature 

 Redia, stands higher in the series than the Orthonectida. What influence these facts have 

 upon our views of the gradual progress of parasitic life how beautifully and naturally 

 they come into accord with the views expressed in text hardly needs any further com- 

 ment. R. L.] 



