GASTEROPODES. 277 



The period does not seem to be confined to any particular season of 

 the year. 



When most regular the spawn consists of about six volutes of small 

 diameter, as appeared in that from a specimen fifteen lines, produced on 

 February 7. Yet there is no positive rule. This latter spawn came to 

 maturity in eighteen days. An example corresponding in all circum- 

 stances occurred seven years afterwards, when the ova came also to ma- 

 turity in eighteen days from November 11. Two or three days usually 

 elapse between the escape of the first and last of the young from their 

 prison. 



The spawn of the whole tribe of the Doris exhibits a multitude of 

 white spherical ova, as they appear amidst the rope, if it be a belt issuing 

 from the parent. Plate XXXIX. fig. 1, enlarged. Each ovum contains 

 the elements of a single embryo, whose evolution in the same form as the 

 parent would be expected by the observer. But as the ovum advances 

 onwards, the motion of an animal apparently different in shape and aspect 

 of the parent is discovered within, as shewn on inspection of a portion 

 of the spawn. Plate XXXIX. fig. 2, enlarged. 



This creature having gained that state of maturity enabling it to 

 quit its position, presents an extraordinary view, for it shews an intimate 

 resemblance to some of the Vorticellae while lodged in the shell. Two 

 cylinders, apparently hollow, and crowned by a fringe, rise from the 

 shoulder of what must be compared to that of other animals, and the 

 body itself seems to contain several viscera. Plate XXXVIII. fig. 13, 

 nascent animals from the spawn ; fig. 14, enlarged ; fig. 15, another more 

 enlarged. 



As the spawn continues dissolving, multitudes of these creatures, 

 like the most minute specks, swim through the water. 



They remain visible and active a considerable time, at length their 

 course relaxes ; their sources of motion are impaired ; they become con- 

 tracted and totally disappear, leaving only a small shell at the bottom of 

 the vessel containing them. 



Having never heard of anything of the kind, I own that on origi- 

 nally seeing these creatures amidst the ova of the Doris, and afterwards 



