12 



General Morphology. 



Since on purely anatomical grounds we are convinced that the ring of attachment of 

 the Rhizocephala equals the stalk of the (impedes it is highly suggestive to tind a normal 

 Pedunculate Cirripede which is on the way to convert the stalk into a similar organ of nutri- 

 tion with a root system branching out if it. It is however more than probable that the actual 

 resemblance of Anelasma to the Ehizocephala is due to convergence, for Anelasma is a fairly 

 typical genus of the Pedunculata in all other respects. 



The parasite of the isopod Ccdathura, which I have named Duplorbis, will have to be 

 taken account of in dealing with the phylogeny of the Rhizocephala, but the discussion of 

 this genus will be introduced more appropriately after dealing with the endoparasitic stages of 

 development, since its structure throws light more especially on the derivation of the endo- 

 parasitism of the Rhizocephala. For further remarks, therefore, on the general relationships 

 and derivation of the Rhizocephala the reader is referred to chapter 4 p. 58. 



After this preliminary survey of the organization of our group we may turn to a more 

 detailed comparison of the various genera, which will reveal to us many interesting deviations 

 both in structure and symmetry. 



B. The inter-relationship of the Genera of Rhizocephala. 



The Rhizocephala, as far as our present knowledge extends, comprise the twelve genera 

 Duplorbis, Clistosaccus, Peltogaster, Parthenopea, Sylon, Sacculina, Heterosaccus , Lernaeodiscus, 

 Triangulus, Thompsonia, Apeltes and Thylacoplethus. Of these Duplorbis, Heterosaccus and Trian- 

 gulus are described for the first time in this work : but I must consider the anatomy of Clisto- 

 saccus, Sylon and Lernaeodiscus as hitherto imperfectly known. The three genera Thompsonia, 

 Apeltes and Thylacoplethus are too slightly characterised to be included in this discussion. 



The other nine genera may be separated from one another chiefly by the condition of 

 the mesentery and the symmetry of the body relatively to that of the host, and a considera- 

 tion of the variations of these characters, combined with that of the general anatomy, may lead 

 us to the conclusion that Peltogaster preserves the most generalized condition, and that the 

 characters, especially those of asymmetry, of the other genera, with the exceptions of Clisto- 

 saccus, Duplorbis and Sylon, are modifications of a primitively Peltogaster-like structure and 

 symmetry. In developing this thesis I am only extending the original idea of Giard (12 and 

 16) applied relatively to Peltogaster and Sacculina. 



The anatomy of Peltogaster has been sketched in the first part of this section. I base 

 my view of its generalized nature upon the compact, simply branching root system, the un- 

 branched and simple oviducts whose walls are not complicated into an elaborate colleteric 

 gland as in the other genera, and upon a feature in the endoparasitic development which consists 

 in the entire absence of a perisomatic cavity, a characteristic organ of Sacculina (see p. 51); 

 but chiefly upon the simple symmetrical relation to its host. 



Peltogaster is parasitic on the Hermit-crabs, and is affixed to the dorso-lateral surface 



