General Morphology. 



13 



of the left side of the abdomen, with the antero-posterior axis elongated in the same direction 

 as that of its host, as shown in Text fig. 4 ; the parasite is curved in correspondence to the 

 curvature of the crab's abdomen; and the paired organs of the Peltogaster, viz.: the vasa defe- 

 rentia and oviducts, are symmetrically disposed about the long axis of the mesentery, the genital 

 openings on the morphological left side of the Peltogaster actual right in Figure 4) being slightly 



Text fig. 4. 



in advance of those on the right see Text fig. 5 A . The symmetry of the body is in fact in 

 complete and constant accord with the symmetry of the host. 



Now all the other genera of Rhizocephala, leaving aside Duplorbis, Clistosaccits and Sylon, 

 are parasitic on the orders of symmetrical Decapoda, and the peculiar feature of their organi- 

 zation is that they all exhibit a constant asymmetry relatively to their hosts, while the genera 

 Lemaeodiscns and Triangulus are not only asymmetrical relatively to their hosts but are in them- 

 selves highly asymmetrical animals. That the position of a parasite on its host should be 

 irregularly asymmetrical and indefinite is natural, but that a whole group should be asymmetrical 

 in a constant and fixed manner calls for some explanation. 



Bearing in mind that these genera in their general organization show a greater degree 

 of complication than Peltogaster, for instance in the root system and in the branching colleteric 

 gland, we infer that they owe their peculiar symmetrical relations to their descent from a 

 Peltogaster-hke form which was parasitic on the asymmetrical Hermit-crabs, and that the act 

 of shifting from a dorso-lateral position on the Hermit-crab's abdomen to a ventro-median 

 position on the abdomen of the symmetrical crabs has led to a distortion of the original bilateral 

 symmetry of the Peltogaster form. Because if we imagine the Peltogaster in Text Figure 4 simply 



