General Morphology. 7 



past. We will therefore consider the comparative anatomy of the Rhizocephala with refer- 

 ence to two problems, fìrstly what is the relationship of the Rhizocephala to other Crostacea, 

 secondly what is the relationship inter se of the different forms of Rhizocephala. The partial 

 solution of these problems will bear a practical importance since it will enable us to determine 

 a rational orientation of the body of these imperfectly understood animals. 



A. The Relationship of Rhizocephala to other Crustacea. 



An examination of the characters of an adult, without limbs or gut and with the ner- 

 vous system reduced to a single ganglion, might leave us in doubt not only of the order of 

 Crustacea to which the Rhizocephala should be referred, but even as to their Crustacean affi- 

 nities at ali: we are however able from the distinctive features of their free-swimming larvae 

 to fix the Rhizocephala as a suborder of the Cirripedia. The shape of the Nauplius with its 

 frontal horns (Piate 4 figs. 17 and 18) and the passage through a Cypris stage (Piate 4 fig. 19), 

 characteristic of Cirripedes, are sufficient for the argument. It may nevertheless be borne in 

 mind that the Nauplius and Cypris larvae of the Rhizocephala differ from those of ali other 

 Cirripedes in the invariable absence of a gut, but this peculiarity is correlated with the sub- 

 sequent endoparasitic development. The endoparasitic stage which follows the free-swimming 

 Cypris stage throws a great difnculty in the way of interpreting the adult anatomy, for this 

 endoparasitic stage practically amounts to the resumption of an embryonic state in which ali 

 continuity with the organs and orientation of the Cypris larva is lost. The adult parasite is 

 evolved from a little mass of undifferentiated embryonic cells which are passed from the body 

 of the Cypris into the body-cavity of the host and after a period of wandering and growth 

 become differentiated into the two essential portions of the adult body, namely the root 

 system which remains inside the host, and the sac-like external portion which is thrust out 

 of the host's body and comes to maturity ectoparasitically. It is this external sac-like portion 

 which carries ali the essential adult organs and which we must attempt to homologise in some 

 manner with the adult Cirripede body, but the discontinuous method of development of this 

 body from the Cypris larva precludes the possibility of founding our homologies on an em- 

 bryonic basis. 



We are therefore compelled to seek in the comparative relations of the adult anatomy 

 itself for some clue as to the orientation and homology of the body relatively to other 

 Cirripedes. 



To illustrate the adult structure of our group I choose Peìtogaster, because for reasons 

 that will be given later, this genus may be considered to retain the simplest and most typical 

 organization of the Rhizocephala. 



The Root system of Peltogaster does not concern us; it forms a compact green mass 

 in the abdomen of the infected Hermit-crab, taking up the position of the gonad. The roots 

 communicate with the external portion of the body at the chitinous ring which affixes the 



