738 SUMMARY OF CURRENT RESEARCHES RELATING TO 



whicli open into the stomach, and though the mucous membrane of these 

 two organs strikingly differs, there is a gradual transition which makes 

 it difficult to mark the boundary between them. The protoplasm of the 

 hepatic cells has a spongy texture ; in section there is revealed a system 

 of large spaces, the trabeculse of which are irregular in appearance. 



]So one has yet described the histology of the nerve-ganglia, the 

 general arrangement of which has been so well explained by Lacaze- 

 Duthiers. They are composed of large ganglionic cells, small ganglionic 

 cells, and fibrillar tissue, all of which are regularly arranged. The two 

 cerebral ganglia are really one ganglion subdivided by a constriction 

 in the sagittal plane ; the substance, which consists of large and small 

 ganglionic cells, extends uninterruptedly from one ganglion to the other ; 

 the tissue with large ganglionic cells forms the cortical layer below and 

 at the sides ; that with small cells is found on the dorsal surface. The 

 fibrillar substance, which corresponds to the white substance in the 

 brain of Vertebrates, occupies the whole of the interior of the ganglia. 

 In the pedal ganglia there is not the same distinction between the two 

 kinds of ganglionic tifc,sue ; the whole of the cortical layer, except at 

 the points of origin of the nerves, is formed by cells of medium size, and 

 the whole of the interior is occupied by the fibrillar substance; the 

 latter alone extends uninterruptedly from one ganglion to the other, and 

 so we have really a pair of ganglia. The other ganglia do not, from the 

 histological point of view, deserve their name ; ganglionic cells are only 

 scattered on their surface, and there is no continuous layer. The author 

 enters into a good deal of detail in this portion of his memoir. 



Histologically, all the muscular fibres of Dentalium, to whatever 

 organ they belong, are of the same structure ; they are very long fibres, 

 oval or circular in section, and rendered more or less polyhedral by 

 mutual pressure. They only vary in length ; the very long fibres are 

 found in the locomotor organs, and the shortest in the region of the 

 intestine. The nucleus of the fibre is always excentric in position ; the 

 amount of granular sarcode which surrounds the nucleus, especially at 

 its extremities, is reduced in the fibres of the adult to a scarcely per- 

 ceptible minimum. The several muscular organs are fully described. 



The blood of Dentalium, is colourless, and contains nucleated cells 

 which recall the white blood-corpuscles of Vertebrates. The state- 

 ment of Lacaze-Duthiers that there is no heart is too absolute. Physio- 

 logically, it is true that Dentalium has no heart comparable to that which 

 propels the blood in higher Molluscs, but, on the other hand, M. Fol 

 shows that the " perianal sinus *' is provided with muscles, which, from 

 the morphological point of view, he cannot imagine any one refusing to 

 regard as the homologue of the heart of other Molluscs. This circum- 

 anal sinus (as it had better be called) has a delicate wall formed by an 

 epithelium with flattened and spread-out cells ; externally to this there 

 are ribbon-like muscular fibres which are generally arranged parallel to 

 one another, and leave between them free spaces which are about four 

 times as wide as a fibre. Most of the fibres are set longitudinally, but 

 a few run across and make angles with the rest. The other sinuses do 

 not seem to possess either muscles or a true endothelial layer, and such 

 are true sinubes. In a transparent Dentalium the author has seen, under 

 the Microscope, the contractions of the circumanal vessel. The question 

 as to whether the simplicity of the cardiac arrangements of Dentalium is 

 acquired (f)r degoncratcfl) or primitive must as yet remain open. 



