In a very considerable number of cases amongst Gasteropods and Pteropods the auditory organs have been observed to develop as invaginations of the epiblast, which give rise to closed vesicles lying in the foot, _e.g._ Paludina, Nassa, Heteropods, Limax, some Pteropods (Clio). This is no doubt the primitive mode of origin, but in other cases, which perhaps require confirmation, the sacks are stated to originate from a differentiation of solid thickenings of the epidermis or of the tissues subjacent to it. The auditory sacks are provided with an otolith, which according to Fol's observations is first formed in the wall of the sack. In Cephalopods the auditory organs are formed as epiblastic pits on the posterior surface of the embryo, and are at first widely separated (fig. 113, _ac_). The openings of the pits become narrowed, and finally the original pits form small sacks lined by an epithelium, and communicating with the exterior by narrow ducts, equivalent to the _recessus vestibuli_ of Vertebrates, and named, after their discoverer, Kölliker's ducts. The external openings of these ducts become completely closed at about the same time as the shell-gland, and the ducts remain as ciliated diverticula of the auditory pits. The widely separated auditory sacks gradually approach in the middle ventral line, and are immediately invested by the visceral ganglia (fig. 124, _ac_). They finally come to lie in contact on the inner side of the funnel. On the side opposite Kölliker's duct, an epithelial ridge is formed--the _crista acustica_--the cells of which give rise to an otolith connected with the crista by a granular material. At a later period of development three regions of the epithelium of the sack become especially differentiated. Each of these regions is provided with two rows of cells, bearing on their free edges numerous very short auditory hairs. The cells of each row are placed nearly at right angles to those of the adjoining row. [FIG. 125. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO AT TWO STAGES. (After Bobretzky.) _hl._ inner segment of lens; _vl._ outer segment of lens; _a_ and _a´_. epithelium lining the anterior optic chamber; _gz._ large epiblast cells of ciliary body; _cc._ small epiblast cells of ciliary body; _ms._ layer of mesoblast between the two epiblastic layers of the ciliary body; _af._ and _if._ fold of iris; _rt._ retina; _r´´._ inner layer of retina; _st._ rods; _aq._ equatorial cartilage.] Muscular system. The muscular system in all groups of Molluscs is derived entirely from the mesoblast. The greater part of the system takes its origin from the somatic mesoblast. In almost all Gasteropod and Pteropod larvæ there is present a well-developed spindle muscle attaching the embryo to the shell. This muscle appears to be absent in the Cephalopoda.