178 ANATOMY AND LIFE HISTORY OP MOLLUSCA. 



depressed lines. Now this portion of the species is covered with 

 innumerable highly refractive dots and points, and with bodies 

 which appear like protruding sense-organs, somewhat similar to 

 the knobs already described on the outer surface of Triyonia. 

 On the very outer edge of the peristome the lenses are 

 symmetrically arranged in rows, and altogether this is the best 

 part of the shell on which these organs are displayed. They are 

 very minute and exceedingly numerous, though they are separated 

 by an interval of thrice their own diameter. 



Like many of the genus the operculum is studded with small 

 transparent warts or protuberances, much larger than anything 

 in the shape of eyes. In the species now referred to they are 

 numerous, gradually increasing in size from the nucleus to the 

 margin, being largest and fewest at the opening of the spiral 

 corresponding to the aperture. The whole surface of these is 

 seen under the microscope to be covered with small hyaline 

 lenses, or else the exit for sense-organs. Some of the eyes in 

 the midst of these bosses are of large size. A section through 

 the operculum shows that every one of these protuberances is 

 supplied with nerve branches contained in a capsule or sheath. 

 It is probable that the glassy granules on so many of the 

 operculums and columellas in this genus are connected as in 

 this species with visual organs. 



Development and Life-history. — The study of the life-history 

 of Australian Mollusca cannot be expected to present any 

 remarkable deviations from the usual types. A few years ago 

 nothing was known about their development except in a very few 

 instances, and as yet a better state of things has not extended to 

 Australia. The reason for this need hardly be explained. The 

 difficulties connected with observations on such matters, though 

 very much exaggerated, are enough to deter most naturalists ; 

 besides the great work of collecting and cataloguing the species 

 we possess, is so far from being finished, that it absorbs all the 

 energies of our zoologists, and gives them no time for the arduous 

 and intricate researches of histology. 



Nevertheless something has been done, though it is but little, 

 and before long it is to be hoped that a good deal of special and 

 interesting discovery in these matters will see the light. In this 

 essay I have so little to offer that it is only with extreme diffidence 

 I place on record the following few scattered observations. 



As already stated the variation in the development between 

 different species is but slight, but still there is some difference, and 

 this will be an incentive to our biologists. As an instance of this 

 I may relate what I have observed amongst the oysters. There 

 is no difficulty of obtaining material for observing the ova of this 

 Mollusk. The ova of all the Lamellibranchiata are developed in 



