432 MR. GEOFFREY W. SMITH ON 



It will be seen from this account that I fully agree with Dr. Max Koll- 

 mann's conception as to the correct orientation of the parasite, according to 

 which the surfaces applied to the host are respectively right and left, and the 

 two hinges of tlie mesentery are called right and left in correspondence. 

 But the unfortunate mistake which I made in describing the testicular 

 openings has obscured from Dr. Kollmann the rotatory movement of the 

 mesentery which has resulted in bringing some of the anterior organs, viz., 

 the nerveganglion/mantle-opening, and one or both oviducts, onto the right 

 side. Dr. Kollmann believes that it is easier to derive Lernceodiscus from 

 SaccuVma or Heterosaccus than from Peltogaster. As a matter of fact, this 

 does not influence the correct orientation of the parasite, since Sacadina is 

 easilv derived from Peltogaster or vice versa, the organs being disposed 

 svmmetrically in relation to the mesentery in both these genera. The 

 peculiar thing about Sacculina is that the symmetry of the parasite does not 

 correspond to that of the host, the mesenterial or dorsal side of the parasite 

 beino- always on the morphologically right side of the host's long axis. It 

 has always seemed to me a curious fact that this should be so, and I am still 

 further puzzled by the hypothesis that Sacculina on infecting the symmetrical 

 Anomura should have given rise to a form like Lernoiodiscus, which has 

 acquired a totally different symmetry both in its own structure and in its 

 relation to the host. As I originally pointed out, if we derive all these forms 

 from a PeItogaster-\ike genus parasitic on the Asymmetrical Pagurids, we can 

 understand that on infecting symmetrical forms of Anomura or Brachyura, 

 the relation of symmetry to the host might very well undergo mai'ked and 

 various changes. The argument however is almost purely speculative, and 

 does not influence our views as to the correct orientation of our parasites. 



We may resume our account of the genus and species of Leriucodiscus as 

 follows : — 



Genus Lern.eodiscus {F. Midler (1)). 



Diagnosis. External body of adult yellow. 



Roots yellow, widely distributed and ramifying, without lagense. 



Mantle highly muscular, and thrown into lappets or folds to a greater or 

 less extent. 



Mnntle-opening situated either in the middle line or more usually deflected 

 to the right side, relatively to the host. 



Mesentery broad, pierced by the peduncle, which separates two hinges; 

 the morphologically right hinge being applied to the thoracic surface, the 

 left hinge to the abdominal surface, of the host. The long axis of the 

 parasite has undergone a peculiar rotation, which has resulted in bringing the 

 nerve-o-anglion and mantle-opening on to the right hinge of the mesentery 

 (see figs. 7 & 8). 



