202 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The pedal ganglia and otocysts arise independently, and the forme 

 arc rather late in appearing ; the first signs of them are well-marked 

 thickenings of the ectoderm, which may he called pedal plates. 



After describing the formation of the pharyngeal commissure and the 

 peripheral nervous system, the author deals with the glands of the foot. 

 Of these thero are, as in other molluscs, two, quite distinct from one 

 another. 



The differentiation of the mesoderm is dealt with in some detail. As 

 in other Ctenobranchs, there is a provisional and a permanent heart. 

 From the morphological point of view, the spaces which are in relation 

 with tho former correspond to the cavities of the circulatory organs of 

 the adult, and are, liko them, the remains of the blastocoel. The author 

 is inclined to think that the embryonic heart has a purely ectodermal 

 origin, and that it is derived from a vesicular invagination of that layer. 

 Its external wall is formed by ectoderm, the cells of which are remark- 

 ably modified, becoming fusiform and elongated in the direction of the 

 long axis of the embryo. Although there is not yet full direct evidence, 

 we may suppose that the cavity of the heart is in communication with 

 those of the velum. The permanent heart is formed from the dorsal 

 unpaired rudiment of the mesoderm. 



The temporary renal organs which are often found in molluscs 

 seem to be absent from Vermetm. The permanent kidney appears very 

 late ; the mass of mesodermal cells from which it arises is at first placed 

 in the anterior ventral angle of the pericardiac cavity. 



The ectoderm gives rise to the oesophagus, and the endoderm to 

 the intestine and rectum. From the first there arise three outgrowths, 

 the median of which forms the radula-sac, and the two lateral the salivary 

 glands. In the formation of the intestine the greater part of the central 

 mass of the endoderm is converted into a nutritive mass ; the boundaries 

 of the cells disappear, the nuclei become swollen, lose their contour, and 

 are resolved into a finely granular substance, which soon fuses with 

 the protoplasm of the cells. 



At the conclusion of this description of the facts which he has 

 observed, Prof. Salensky proceeds to consider the formation of the 

 embryonic layers of Vermetus in connection with the ancestral form 

 of the Metazoa. He is not satisfied with the Gastrula of Haeckel, the 

 Planula of Ray Lankester, the Parenchymula of Mecznikow, or the 

 Plakula of Butschli. He believes that the first differentiation which 

 characterizes the passage of the colonial Protozoa to the prototype of 

 the Metazoa consisted in the division of cells into motonutritive and 

 genital ; this idea was simultaneously conceived by Gotte and the author ; 

 it has a close relation to the question of the origin of the blastopore. 

 The difficulty of the disappearance of this orifice, and the appearance 

 of secondary ones, cannot be explained by physiology, but the facts 

 become readily comprehensible if we admit that the ancestral form of 

 the Metazoa must be sought for in vesicular colonies of Flagellates, 

 developing on the type of the Volvox of the present day ; the blastopore 

 would correspond to the orifice seen in young colonies of Volvox, and it, 

 we know, disappears in the course of development. The hypothetical 

 organism for which we are searching would differ from the actual 

 Volvox, in that the cellular individuals of the colony would be capable 

 of animal nutrition, such as is probably the case in Proti>spongia. Like 

 Volvox, the hypothetical ancestor of the Metazoa would be capable of 



