In the case of Paludina the mesoblast becomes two layers thick, and then splits into a splanchnic and somatic layer, of which the former attaches itself to the hypoblast, and gives rise to the muscular and connective-tissue wall of the alimentary tract, and the latter attaches itself to the epiblast, and forms the muscular and connective-tissue wall of the body and other structures. The two layers remain connected by protoplasmic strands, and the space between them forms the body cavity (fig. 107). In most instances there would appear to be at first no such definite splitting of the mesoblast, but the layer has the form of a scattered network of cells between the epiblast and the hypoblast. Finally certain of the cells form a definite layer over the walls of the alimentary canal, and constitute the splanchnic mesoblast, and the remaining cells constitute the somatic mesoblast. [FIG. 101. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. (From Lankester.) _f._ foot; _ot._ otocyst; _m._ mouth; _v._ velum; _ng._ nerve ganglion; _ry._ residual yolk spheres; _shs._ shell-gland; _i._ intestine.] We must now return to the embryo at the time when the blastopore is becoming narrowed. First of all it will be necessary to define the terms to be applied to the various regions of the body--and these will best be understood by taking a fully formed larva such as that represented in fig. 101. The ventral surface I consider to be that comprised between the mouth (_m_) and the anus, which is very nearly in the position (_i_) in the figure. As a great protuberance on the ventral surface is placed the foot _f_. The long axis of the body, at this period though not necessarily in the adult, is that passing through the mouth and the shell-gland (_shs._): while the dorsal surface is that opposite the ventral as already defined. Before the blastopore has attained its final condition three organs make their appearance, which are eminently characteristic of the typical molluscan larva. These organs are (1) the velum, (2) the shell-gland, (3) the foot. The velum is a provisional larval organ, which has the form of a præoral ring of cilia, supported by a ridge of cells, often in the form of a double row, the ventral end of which lies immediately dorsal to the mouth. Its typical position is shewn in fig. 101, _v_. There are considerable variations in its mode and extent of development etc., but there is no reason to think that it is entirely absent in any group of Gasteropoda or Pteropoda. In a few individual instances, especially amongst viviparous forms and land Pulmonata, it has been stated to be absent. Semper (No. 274) failed to find it in Vitrina, Bulimus citrinus, Vaginulus luzonicus, and Paludina costata. It is very probably absent in Helix, etc.