84 SECTIONAL ADDRESSES. 



The gap between Amphineura and Conchifera is absolute : the shell 

 in the former consists of a series of plates (or spicules), in the latter 

 of a single plate (calcified from two lateral centres in Bivalvia). No 

 Amphineuran, living or fossil, approaches the Conchifera by showing an 

 enlargement of one of its plates as the possible predecessor of a single 

 shell, and no Conchiferan, living or fossil, approaches the Amphineura by- 

 showing any signs of a duplication or segmentation of its shell into 

 metameric plates. 



Similarly in Conchifera, whether the Nautiloid or the conical shell be 

 regarded as primitive, no Cephalopod shows any signs of a lateral torsion 

 approaching the Gastropod twist, and no Gastropod exists with paired gills, 

 auricles and kidneys without also displaying a complete torsion of its 

 mantle-cavity and shell through 180° from back to front. The same 

 peculiarity marks off the Gastropoda absolutely from the Scaphopoda and 

 Bivalvia, although in other respects the morphological agreement between 

 these three orders is extensive and detailed and their relationship must 

 be exceedingly close. 



Now let us turn to the larval history of a primitive Gastropod, say 

 Patella' or Trochus,^ and see how this torsion is accomplished. The 



Fig. 4. — Larval Stages of Patella, 



trochosphere develops a cap-like shell upon its back, and swims about 

 with it. As the mantle grows more rapidly behind than in front, the 

 additions to the shell are also more extensive behind than in front, so 

 that, relatively to the newer and broader part, the original ' cap ' is slowly 

 and steadily pushed upwards and forwards as the apex of a commencing 

 coil. This coil of the larval shell is not quite median in existing forms, 

 but there is reason to believe that it was so in the earliest Gastropods, as 

 shown by the symmetry of the shells in practically all Gastropods known 

 from Cambrian and Ordovician strata (e.g. Cyrtolites, Sinuites, Salpingostoma, 

 Bellerophon). Thus the larval shell grows like that of the Pearly Nautilus 

 and is at first orientated in the same way : the apex of its coil is directed 

 upwards and forwards over the larval head (exogastric), and a gill-chamber 

 is developed beneath it behind, corresponding to that of Chiton and 

 Nautilus. At this stage the foot projects freely but carries no operculum ; 

 and it is easy to see from the arrangement of parts that an operculum on 

 the foot would be meaningless. For the head is separated from the gill- 

 chamber behind by the whole length of the foot, and, when the body 



° Patella, Patten, Wien Arbeiten, VI, 1886 ; Acmoea, Boutan, Arch. Zool. Exp., 

 (3) VII, 1899. 



^ Trochvs, Robert, Arch. Zool. Ea-p., (3) X, 1903 ; and Zoologie Descriptive 

 II, 1900, fig. 508. 



