4. The general nature of the endoparasitic development of the Rhizocephala. 59 



stage consists of undifferentiated cellular elements, the ovary and other organs 

 being a later differentiation from them. 



It appears therefore that there is even less continuity between the structure of the 

 Cypris and of the adult than Delage supposed, and that a truly embryonic undifferen- 

 tiated period is interpolated in the life-history between the Cypris larva and 

 the adult. 



The embryonic cells of the Cypris and of the Kentrogon are paralleled by those 

 cells of the Rediae of Trematodes which give rise to Cercariae, except that in the 

 Khizocephala only one adult is produced from a single Cypris. And yet we have seen that 

 there is a tendency in the occasionai polyembryony of Sacculina to simulate stili further the 

 condition of the Trematodes, and it was further suggested that the so-called gregarious forms 

 of the Rhizocephala might be due to the production of several adults from the embryonic 

 cells of a single Cypris. However this may be, the remarkable life history of the Rhizo- 

 cephala shows an intermediate condition of a most suggestive kind between an ordinary con- 

 tinuous development and a trae alternation of generations. 



It is perhaps idle to speculate why the Rhizocephala have acquired a developmental 

 history so utterly different from that of ali other Crustacea. We may however safely suppose 

 that the ancestors of the Rhizocephala were originally ectoparasites on other Crustacea, and 

 that owing to the dangers to the parasites involved by the continuai moulting of their hosts, 

 they gradually acquired a partially endoparasitic habit. Hand in hand with this acquisition, 

 an economy was practised in the division of the developmental phases into two periods, firstly 

 the rapid establishment of the root system, and subequently the retarded differentiation of the 

 adult organs. The chief advantage of this postponement of the differentiation of the adult 

 organs is perhaps to be found in the prevention by this means of the parasite being interfered 

 with by the host's moulting: for when the retarded development of the adult organs takes place 

 and the part of the body containing the reproductive organs must be protruded from the 

 host for the purposes of reproduction, the precocious establishment of the root system has 

 inhibited the growth of the host, and prevents the continuai moulting which would certainly 

 interfere with the welfare of the parasite. In this manner we may perceive the usefulness 

 of the interpolation of an endoparasitic undifferentiated state for the etablishment of the root 

 system, but as to how the developmental processes are so shifted we are as ignorant in this 

 case as in every other. 



As far therefore as the adaptational meaning of the process is concerned, the clue 

 appears to He in the necessity for the parasite to overcome the dangers incident to parasitism 

 on a Crustacean which is continually casting its skin. In Sacculina, which from every point 

 of view we regard as the most highly specialized of the Rhizocephala, the adaptational relation 

 of the parasite to the moulting of its host has reached a point of extreme delicacy. For by 

 the time the root system has been established and the adult organs are beginning to develope 

 in the proper situation, the growing-powers of the host have not been so completely inhibited 



8* 



