Gastropoda. 



21 



features are really secondary. The Gastropoda are divided into 



two sub-classes, the Streptoneura, in which twisting of the 

 visceral commissure is well marked, and the Euthyneura, in 

 which untwisting has taken place, in the Streptoneura the 

 nervous ganglia are relatively little concentrated, whereas in the 

 Euthyneura they are, as a rule, much concentrated in the head 

 region. A structure present in most Streptoneura, but in only a 

 few Euthyneura, is the operculum, a hard plate borne on the upper 

 surface of the foot, serving to close the aperture of the shell when 

 the animal is withdrawn. In the majority of the Streptoneura 

 the sexes are separate, but all Euthyneura are hermaphrodite. 



Fig. 11. 



The Common Rock-Limpet (Patella vvlgata). British. 



1. Animal: a. foot; b. fringed mantle; c. tentacles; d. mouth; e. eyes; 



/• gills- 



2. Side view of shell, showing the impression or scar of the attachment - 

 muscle, a. 



3. Uf>per surface of the shell. 



Sub-class I. STREPTONEURA. 

 Order 1. ASPIDOBRANCHIA. 



{Cases 3 to 20.) 



This order includes Gastropods with paired auricles of the heart, 

 paired kidneys, and occasionally paired gills, and with the gill- 

 filaments set on opposite sides of a main axis. The first sub-order, 

 the Docoglossa, is so called because the teeth of the radula have 

 the shape of beams. It includes the Limpets (Acmaeidae and 

 Patellidae), in which the shell is not spirally coiled. 



The Acmaeidae are called ' false Limpets ', because, although 

 their shells are similar to those of the true Limpets, they have 

 a single true gill, whilst the Patellidae have false gills greatly 

 developed as outgrowths of the mantle all round the sides of the 



