Although the majority of important developmental features are common to the whole of the Mollusca, yet at the same time many of the subdivisions have well-marked larval types of their own. It will for this reason be convenient in considering the larval characters to deal successively with the different subdivisions, but to take the whole group at once in considering the development of the organs.
Formation of the layers and larval characters.
Odontophora.
Gasteropoda and Pteropoda. There is a very close agreement amongst the Gasteropoda and Pteropoda in the general characters of the larva; but owing to the fact that the eggs of the various species differ immensely as to the amount of food-yolk, considerable differences obtain in the mode of formation of the layers and of the alimentary tract.
The spheres at a very early stage of segmentation[100] become divided into two categories, one of them destined to give rise mainly to the hypoblast, the other mainly to the epiblast. According as there is much or little food-yolk the hypoblast spheres are either very bulky or the reverse. In all cases the epiblast cells lie at one pole, which may be called the formative pole, and the hypoblast cells at the opposite pole. When the bulk of the food-yolk is very great, the number of hypoblast spheres is small. Thus in Aplysia there are only two such spheres. In other cases, where there is but little food-yolk, they may be nearly as numerous as the epiblast cells. In all these cases, however, as was first shewn by Lankester and Selenka, a gastrula becomes formed either by normal invagination as in the case of Paludina ([fig. 107]), or by epibole as in Nassa mutabilis ([fig. 105]). In both cases the hypoblast becomes completely enclosed by the epiblast. The blastopore is always situated opposite the original formative pole. In the large majority of cases (i.e. Marine Gasteropoda, Heteropoda, and Pteropoda) the blastopore becomes gradually narrowed to a circular opening which eventually occupies the position of the mouth. It either closes or remains permanently open at this point. In some cases the blastopore remains permanently open and becomes the anus. The best authenticated instance of this is Paludina vivipara, as was first shewn by Lankester (No. [263]).
In some instances the blastopore assumes before closing a very narrow slit-like form, and would seem to extend along the future ventral region of the body from the mouth to the anus. This appears, according to Lankester (No. [262]), to be the condition in Lymnæus, but while Lankester believes that the closure proceeds from the oral towards the anal extremity, other investigators hold that it does so in the reverse direction. Fol (No. [249]) has also described a similar type of blastopore. In an undetermined marine Gasteropod, with an embolic gastrula, observed by myself at Valparaiso, the blastopore had the same elongated form as in Lymnæus, but the whole of it soon became closed except the oral extremity; but whether this finally closed could not be determined. It is probable that the typical form of the blastopore is the elongated form observed by Lankester and myself, in which an unclosed portion can indifferently remain at either extremity; and that from this primitive condition the various modifications above described have been derived[101].
Before the blastopore closes or becomes converted into the oral or anal aperture, a number of very important embryonic organs make their appearance; but before describing these it will be convenient to state what is known with reference to the third embryonic layer or mesoblast.
This layer generally originates in a number of cells at the lips of the blastopore, which then gradually make their way dorsalwards and forwards, and form a complete layer between the epiblast and hypoblast. The above general mode of formation of the mesoblast may be seen in [fig. 107], representing three stages in the development of Paludina.
In some cases the mesoblast arises from certain of the segmentation spheres intermediate in size between the epiblast and hypoblast spheres. This is the case in Nassa mutabilis, where the mesoblast appears when the epiblast only forms a very small cap at the formative pole of the ovum; and in this case the mesoblast cells accompany the epiblast cells in their growth over the hypoblast ([fig. 105]).
In other cases the exact derivation of the mesoblast cells is quite uncertain. The evidence is perhaps in favour of their originating from the hypoblast. It is also uncertain whether the mesoblast is bilaterally symmetrical at the time of its origin. It is stated by Rabl to be so in Lymnæus[102].