218 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



suggests that the segmentation of Platyhelmintha, Annelida, Mollusca, and 

 Molluscoida can be referred to a common type. It follows that the regular 

 segmentation occurring in certain forms in each of these groups is brought 

 about secondarily by loss of yolk originally present. 



3. The velum. — This organ is developed from paired ectodermic folds on 

 the ventral surface, which meet neither dorsally nor ventrally. The author 

 finds in many Gastropods, that the velum consists of a preoral band of 

 strong cilia, and a postoral band of smaller cilia ; and between the two an 

 area clothed with fine cilia, continuous with those of the oesophagus ; this 

 arrangement is probably characteristic of Prosobranchs. 



4. Excretory organs. — In Fuhjur there is no "head-kidney," but the 

 larval kidney consists of ectoderm cells with highly refractive contents, as 

 in Nassa. 



Paludina and Bithynia alone amongst Prosobranchs possess a " head- 

 kidney " as well as a larval kidney. The secreting cells were probably 

 originally part of a preoral velar area, and as they became more important 

 they separated from it and eventually replaced the " head-kidney." 



5. Nervous system and sense-organs. — The cerebral and pedal ganglia are 

 developed in the usual way from paired ectodermic thickenings. The latter 

 have no connection with the " byssus-gland " or " aquiferous pore " as has 

 been stated by some authors. 



The typical apical thickening, as the origin of cerebral ganglia in the 

 ancestor of Annelida and Mollusca, or " Trochozoon," is not present in 

 Prosobranchs ; it is present in Pulmonata, but the develoj)ment of marine 

 forms has been abbreviated and thus has been lost. 



At the end of each chapter a summary and discussion of the results of 

 previous writers is given. 



Typical Nervous System of Prosobranchs.* — M. E. L. Bouvier finds 

 that the nervous system of Prosobranchs is characterized by a chiastoneurous 

 visceral commissure, or one twisted in the figure of 8. It has its origin 

 in the commissui"al ganglia and contains a subintestinal branch which 

 arises from the left commissural ganglion and passes backwards from left 

 to right under the cesojjhagus ; it forms a subintestinal ganglion, and 

 trends towards the heart on the right side of the body ; there is also a 

 Bupra-intestinal branch which arises from the right commissural ganglion, 

 and passes from right to left, forming a supra-intestinal ganglion ; it makes 

 its way along the left side of the body to join the subintestinal branch. 

 This is the typical arrangement, but it becomes much more complicated in 

 a number of forms. As to these the author sketches the characters of a few 

 of the more important and significant. 



Lepidomenia hystrix.t — MM. Marion and A. Kowalevsky give an ac- 

 count of a new genus of Solenogastres or amphineural molluscs, which was 

 found at a depth of 30 metres in the Gulf of Marseilles ; a single individual 

 was found in the calyx of a BalanojjJiyllia ttalica, and was scarcely • 002 ra. 

 long. It is allied to Proneomenia by its internal organization, but is 

 strongly distinguished by a very original spicular investment ; the body is 

 entirely covered with strong spines, the bases of which are applied directly 

 to the hypodermis, without the interposition of any cuticular mass, such as 

 is found in Proneomenia. When examined under low powers the surface 

 seems to be covered not by spines, but by imbricated scales ; this apjiear- 

 ance is due to the bases of insertion of the spines which are more apparent 

 than their hyaline mass. The hypodermis is relatively rather thick, and 

 the most numerous of its elements are prismatic cells with large nuclei ; 



* Cumptes Rendus, ciii. (ISSG) pp. 1274-C. f Ibid., pp. 757-9. 



