General Morphology. •[ "J 



ring of attachment to the mantle opening is absent, but the genital openings, although slightly 

 shifted, have the sanie asymmetrical position as in Lernaeodiscus. 



In the above manner, namely by the supposition of their twofold descent from a Pelto- 

 yaster-like forni, I would account for the curious fact that these genera of Rhizocephala, 

 although parasitic on symmetrical crabs, exhibit a Constant and definite type of asymmetry 

 r elati vely to their hosts. 



We have stili to assign a position to the remaining three genera Duplorbis, Clistosaccus 

 and Sylon. The genera which we have considered ali agree closely in their general anatomy, 

 but these three genera show striking differences. Duplorbis possesses so many peculiar features 

 as almost to justify its removal to a separate sub-class. Clistosaccus does not possess a mantle 

 opening, at any rate when adult, while Sylon has two; Clistosaccus has a single median testis, 

 and Sylon does not possess a testis at ali, as I have determined on four specimens by means 

 of serial sections. Both Sylon and Clistosaccus have a median unpaired oviduct. The chief 

 peculiarity of Duplorbis consists in its method of attachment to its host and the apparent absence 

 of a root system, but this forni will be fully discussed at the end of Chapter 4 p. 60. 



In regard to symmetry, Duplorbis, which is highly peculiar in being parasitic on the 

 Isopod Calathura, is immensely laterally elongated, so that the actual long axis of the body 

 is at right angles to the morphological antero-posterior long axis. The position on the ventral 

 surface of the host is indefinitely oblique (Piate 8 fig. 9). 



Clistosaccus (Piate 8 figs. 5 — 8) is, like Peltogaster, a parasite of the Hermit-crabs ; its 

 position on the host is also the same as in Peltogaster. Sylon (Piate 8 figs. 1 — 4) is parasitic 

 on the Shrimps, and is the single genus of the Rhizocephala, parasitic on a symmetrical host, 

 which exhibits a simple symmetrical relation to it. The mesentery runs in the middle line 

 from the ring of attachment to the doublé mantle opening, the antero-posterior axis coinciding 

 with that of the Shrimp. 



Combined with these morphological differences which separate Duplorbis, Clistosaccus 

 and Sylon from the other genera, we find striking differences in the sexual relations, a subject 

 which will be treated fully in Chapter 2. Peltogaster, Sacculina, Lernaeodiscus and the allied 

 genera are structurally hermaphrodite, the testes showing a peculiar kind of degeneration of 

 certain cells. Complemental males in these genera, when present, are entirely degenerate, 

 and do not develope beyond the Cypris stage. In Clistosaccus, Duplorbis and Sylon we have 

 three different modes of reproduction. In Clistosaccus, which is structurally hermaphrodite, the 

 testis shows no signs of degeneration at ali and is crammed with sperm at maturity; in Duplorbis, 

 bodies which may possibly be interpreted as fully developed complemental males containing 

 masses of sperm may be present in the mantle cavity and serve for fertilization ; in Sylon, the 

 animai is structurally female, and since there is no evidence of the occurrence of males, we 

 must presume that parthenogenesis occurs. 



To summarise ali the foregoing facts in tabular form we may construct a kind of genea- 

 logical schenie, as follows, taking a hypothetical Peltogaster-ì\ke ancestor as the centrai form 



Zool. Station zu Neapel, Fauna und Flora, Golf von Neapel. Rhizocephala. 3 



