ON THE STRUCTURE OF ENIGMA ^NIGMATICA. 291 



This rectum, which is clearly proctodasal iu origin, is lined by 

 an epithelium composed of clear and very attenuated ciliated 

 cells with deeply-staining nuclei. 



The whole of the large intestine, and a section of the small 

 intestine preceding it, is infested by sporozoan parasites, 

 whose characters are shown in fig. 24. They are not sufii- 

 ciently well preserved to admit of careful description, but 

 they are evidently the trophozoites of a Coccidian, some of 

 which are forming cysts containing sporoblasts. They have 

 some likeness to the genus Klossia, but as I have not been 

 able to discover the spores, or to trace the various stages of 

 the life-history of this parasite, it will be better to record its 

 existence without conferring upon it a new and probably a 

 misleading name. 



SuMMAEY OP Results. 



X. yEnigma, though modified in the same direction as 

 Auomia, has undergone a less degree of torsion, and has 

 retained more of the typical features of a normal Lamelli- 

 branch. 



2. There is, on the left side, a specialised pallial muscle, 

 attached to the left valve, and acting as a retractor of the 

 left gill. 



3. A ring of eye-spots, of peculiar structure, is found on 

 the left mantle lobe, at a considerable distance from the edge 

 of the mantle. 



4. Adaptations for resisting desiccation during long ex- 

 posure to the sun and air are found in the thickening and 

 corrugation of the lower moieties of the mantle lobes and in 

 the existence of csecal extensions of the pallial cavity, which 

 can be closed by the apposition of the ciliated edges of ridges 

 developed on the mantle and body-wall. 



5. The structure of the byssus gland is of the same type as 

 that of Area tetragona and Anomia ephippium. There 

 is no calcified ossicle. 



6. The inner demibranch of the right gill is attached to 



