772 RECORD OF CURRENT RESEARCHES RELATING TO 



tlie gill, which, at about the same time, becomes marked off from the 

 inner mautlo-lamella by the ingrowing of the fold. 



After some considerations on the early appearance of the bilateral 

 ai-rangement, to which attention has been called above, the author 

 says that, with the exception of the Echinodermata, the Trocho- 

 zoon appears to be the primitive form for all the rest ; WormSj 

 Molluscs, Molluscoids, Arthropods, and Vertebrates may therefore 

 be distinguished as Euhilateria. The blastopore closes along the 

 middle line, and the mouth appears at the point at which lay its 

 final remnant ; the formation of an ectodermal fore-gut appears 

 to have happened very early, and, after this, the formation of the 

 mesoderm is the oldest phenomena. The mode of development of 

 the mesodermal organs is a matter of great interest ; in the Annelids 

 the differentiation of the mesodermal bands leads to the distinction 

 between the head and trunk ; the relations between the trochophore 

 and the Teredo-larva are so close that their common ancestry is not 

 to be doubted ; the early development of the shell is only another 

 examj)le of the appearance before its historic time of an organ which 

 plays an important part in the organization of the individual. 



When we try to trace the phylogenetic history of the mollusc, we 

 see that there were added to the organs of the Trochozoon, first, the 

 ventral ganglion of the trunk with the auditory vesicles, the paired 

 trunk-kidneys, opening by special ciliated infundibula into the 

 secondary coelom ; and the dorsal heart. These organs characterize 

 the primitive ancestor of both annelids and molluscs ; then, for the 

 mollusc, there aj^peared the hepatic diverticula of the stomach, the 

 dorsal shell, the mantle-fold, the muscular foot, and the primary gills. 

 When the foot appeared, the free-swimming mode of life was lost, 

 and the velum began to atrophy. If this be really the true history 

 of the Mollusca, it is clear that the " step-ladder " form of the ventral 

 ganglia (Ihering) cannot be regarded as an indication of a pre- 

 existing segmentation. The lateral approximation of the pedal ganglia 

 is a secondary character, and so, much more, is the approximation 

 and final fusion of these centres with the oesophageal ganglion. 



Do the facts of development as now known to us support the 

 monophyletic or the polyphyletic (Ihering) theory of the history 

 of the Mollusca ? Hatschek believes that the ventral ganglia took 

 their origin from an ectodermal thickening on the ventral side of the 

 trunk-region, and that their approximation to the oesophageal ganglion 

 in the Nudibranchiata is the result of a secondary process. Ihering 

 would think that in (his) Platycochlides the supra-oesophageal, as well 

 as the pedal ganglia, had their origin in the frontal plate. Further 

 investigation of known facts, and further study into still unexplored 

 regions, can alone decide what answer is to be given to these two 

 questions. 



Molluscoida. 



Development of Lingula.* — M. Joliet has an analysis of 

 Mr. W. K. Brooks's important contribution to this subject, to 



* ' Avfli. Zool. Exp. ct Gc'u.,' 1880, p. 390. 



