THE SNAIL 87 



appearance. In those gastropods in which the anterior part of 

 the blastopore persists as the mouth, an invagination of the epi- 

 blast, known as the stomodseum, carries the primitive mouth 

 deeper into the body, and an outgrowth from the ventral wall 

 of the stomodaeum marks the future radula-sac. In those 

 forms in which the anterior end of the blastopore closed up, 

 the posterior part remaining as the anus, a similar stomodaeal 

 invagination grows inwards towards the gut, and fusing with it 

 gives rise to the permanent mouth. The anus, if not previously 

 present, has been formed by an outgrowth of the gut, which 

 met and fused with an epiblastic invagination near the anal tuft 

 of cilia. In the velum a pair of eyespots has been formed 

 right and left of the apical tuft of cilia, and in connection with 

 the eyespots is a pair of epiblastic thickenings representing the 

 cerebral ganglia. A similar pair of epiblastic thickenings re- 

 presenting the pedal ganglia have been formed on the surface 

 of the foot at a little distance behind the mouth, and connected 

 with these is a pair of invaginations which will give rise to the 

 otocysts. The pleural and visceral ganglia do not become 

 apparent till a later stage. In the streptoneurous gastropods the 

 larva, on attaining this stage of development, projects its foot 

 and velum far out of the shell, and twists them round through an 

 angle of 180. In this movement the relative positions of foot, 

 mouth, and velum are not altered, nor are the relations of the 

 organs contained in the visceral mass, but the intermediate part 

 of the body is twisted in such a manner that the anus, which 

 previously was on the ventral side, now becomes dorsal, and 

 the rounded extremity of the shell which previously inclined to 

 the dorsal and anterior side is now directed posteriorly and 

 ventrally. The change only occupies a few minutes, but the 

 torsion is permanent, and it is by this remarkable and rapid 

 metamorphosis that the symmetrical larva is transformed into 

 the asymmetrical adult.* It seems probable that the primordia 

 of the pleural and visceral ganglia and connectives, though 

 not distinguishable, must be localised in the epiblast previous to 

 the metamorphosis, for otherwise it would not be possible to 

 account for their share in the torsion. 



The further development of the larva into the adult form 

 will be best understood by a study of fig. 20, Kto IV. The foot 



* See L. Boutan. Asymetrie des Mollusques Gastropodes ; Archives de 

 Zoologie experimentale et generale. 3 me Ser. vol. vii. 1899. 



