Development of the Pectinihranchiata. 435 



capsule we have seen embryos produced by the union of 3-4 

 eggs, wliilst GO or more have cooperated in the formation of 

 the majority of the individuals. The difference in the size of 

 the individuals also depended upon the same cause. Their size 

 \aried considerably, and embryos of :|^ to 1 millim. were seen 

 moving in the licpiid then contained in the capsule. As the 

 size of the embryos was variable, so was their number. This 

 depended on the greater or less number of eggs which had co- 

 operated in the formation of each individual. We have found, 

 on the average, from 20 to 40, rarely more. 



After the formation of the ciliary membrane, the organs of 

 motion and the foot are the parts which first make their ap- 

 pearance. Nearly at the same time we see between the mem- 

 brane and the conglomerated eggs a transparent and finely 

 granular mass. In this are developed cells, which unite in layers 

 with the above-mentioned membrane, and give origin to the 

 mantle. The lowest part of the latter secretes a tolerably clear 

 and glutinous liquid, which increases gradually, and forms the 

 rudiments of the shell ; this at its first appearance resembles a 

 perfectly clear and gelatinous membrane, in which calcareous 

 particles are afterwards de])osited. These gradually become 

 compact, and in this way render the subsequent investigations 

 diflicult. The rotatory organs are small at their first appearance, 

 but their volume increases by degrees, so that they acquire 

 more and more the form of a funnel. A multitude of cilia 

 a])pear on their surface, and cirrhi make their appearance at 

 their superior margin, and cause far more lively movements. 

 In the foot, which is now clearly detached from the rotatory 

 organs and has acquired a nearly square form, a mass of cells 

 furnished with a nucleus makes its appearance. The margins 

 are almost always inclined towards the ventral part of the ani- 

 mal, giving it a striking resemblance to a hollow cylinder; and 

 as all its surface and its margins are furnished with cilia, the 

 whole acquires the appearance of a ciliated canal. It increases 

 rapidly in volume, and exhibits at its base the first rudiments of 

 the auditory organs, which are formed as in Buccinum. At the 

 same time we also observe the two pyriform salivary glands, 

 which are placed close to the foot, of a yellowish colour, and 

 filled with a quantity of round cells and pigment-grains. When 

 the development has advanced a little further, the tentacles ap- 

 pear in the form of two conical eminences, at the base of which 

 the eyes are discovered in the form of rounded vesicles, filled with 

 a liquid as clear as water, in which some obscure pigment-grains 

 are found. We could not discover any lens in this stage of 

 development. 



On the twenty-third day we discovered the heart. It is formed 



28* 



