5. The morphology of Duplorbis and the derivation of the endoparasitic development. RQ 



Whether Duplorbis actually passes through the stages indicated can only be proved by- 

 future investigation of this genus, but the nature of its fixation and the presence of the 

 mesenteric canal can be explained on that supposition. 



Taking therefore into consideration, firstly the morphological certainty that the Rhizo- 

 cephala have always affixed themselves to their hosts by means of a peduncle = the antennae 

 (see Note p. 62), secondly that in such a form as Anelasma this peduncle, used as an organ of 

 nutrition, is buried in the tissues of its host, and that the organs of the body show a tendency 

 to be pushed up into the peduncle, thirdly that in Sacculina and probably all Rhizocephala 

 the endoparasitic position is attained by the embryonic cells of the Cypris passing right through 

 the antenna (= the peduncle) to gain the interior of the host, and fourthly that in Duplorbis, 

 an undoubted though aberrant Rhizocephalon, fixation is effected by a peduncle, which through 

 its connection by means of a tube with the mantle cavity suggests the process of invagination 

 explained above, — taking all these facts into consideration I think that we are in a position 

 to maintain that the manner in which the endoparasitism of the Rhizocephala has been phylo- 

 genetically acquired, consisted in the penetration of the host's body by the peduncle = the 

 antenna, and by the organs of the body being pushed up into the peduncle so that the whole 

 organism became an endoparasite. Simultaneously or subsequently the root system was added 

 to make the peduncle a more efficient organ of nutrition, and as this root system became of 

 more and more importance in the life history of the parasite it was developed earlier and 

 earlier until finally the condition of indefinite fixation and the simulation of a process of 

 alternate generations was acquired by the higher genera of Rhizocephala. In this manner we 

 may perhaps gain some shadowy idea of the steps by which a process of development, perhaps 

 the most wonderful in the whole field of embryology, has been acquired, and although I do 

 not hold that the riddle is by any means solved, yet there is hope that the discovery and 

 investigation of intermediate forms like Duplorbis may in a short time give us a satisfactory 

 solution. 



I conclude from the apparent absence of a root system in Duplorbis that the Cypris 

 of this animal fixes on the host in the position where the adult parasite is situated. The 

 most peculiar fact in the morphology of the other Rhizocephala e. g. Sacculina, in which the 

 Cypris may fix at any point, is that the adult parasite, although it has lost all connection 

 with the original organ of fixation, yet becomes finally fixed to the host by a peduncle 

 which preserves the essential morphological relations of the Cirripede stalk, as shown in 

 Chapter 1 . 



We may perhaps gain some insight into this remarkable fact by comparing it with 

 that kind of development which Giard has called Poecilogonie (La Poecilogonie. in: 

 Bull. Sc. France Belg. Tome 39 1905 p. 133), in which two closely allied animals, which 

 are almost identical in the adult state, may yet pass through totally different embryonic stages. 

 Thus the adult Rhizocephala are comparable in all their parts with an ordinary adult Cirri- 

 pede, but the two kinds of adult reach their final essential similarity after passing through 



