GASTROPODA. 383 



been stated, and the balance of evidence is against the existence of 

 such openings. 



Metamorphosis of the larva into the adult. However sym- 

 metrical the adult may appear to be, the larva (except in the 

 Ghitonidae) always presents, after the trochosphere stage, an 

 asymmetrical development of its visceral sac, and there is, in 

 almost all, a shell more or less nautiloid in character, and generally, 

 though less invariably, an operculum. In the great majority of 

 EutJiyneura the shell is lost in the adult, and the operculum, with 

 a very few exceptions, also disappears in the same group. In some 

 cases the larval shell is replaced by a permanent shell. 



The asymmetry sometimes appears before the anus and mantle-cavity 

 are formed, in which case the visceral sac becomes more developed on 

 the left side, and the anus and mantle-cavity appear on the right side 

 and then, by the relative growth of parts, shift forwards along the 

 right side until they are placed on the anterior face of the visceral 

 sac. In cases in which the anus appears before the asymmetry, 

 e.g., Paludina, it and the mantle-cavity are at first posterior and 

 median ; they then gradually pass round to the right side, and 

 so to the anterior face of the visceral sac, this shifting being accom- 

 panied by a greater growth of the visceral sac towards the left. 

 This movement of the anus and mantle-cavity, if viewed from the 

 dorsal surface, has been in a direction opposite to that of the hands 

 of a watch, and has consisted of a twisting of the visceral sac 

 about an axis, which may roughly be described as dorso -ventral, 

 through an angle of 180, so that the original posterior face of the 

 visceral sac becomes the actual anterior face, and the visceral organs 

 of the original right side become placed on the left, and vice versa. 

 But in all Gastropods, except the Aspidobranchiata, the organs of the 

 original left side of the visceral sac are either not developed, or, 

 if they are developed, disappear; consequently the anus, instead of 

 opening into the middle of the mantle-cavity, is on its right side, and 

 the actual right gill, osphradium, kidney, and auricle are not present. 

 This torsion of the visceral sac round a dorso -ventral axis in a 

 direction opposite to that of the hands of the watch when viewed 

 from the dorsal pole, is not the only twisting the visceral sac under- 

 goes. It also twists round a horizontal axis passing from right to 

 left in such a manner as to form a spiral. If this twisting be 

 supposed to have taken place before the torsion first described, the 

 visceral sac must have fallen forwards, and so formed a coil like that 

 of the shell of Nautilus on the anterior side: such a coil is called 



