GASTEROPODA. 417. 
where the radula has disappeared, and the head region remains un- 
developed. As already seen, however, the lowest Lamellibranchs 
have a flattened creeping foot and simple feathery gills, in these respects 
resembling Gasteropods. There is also much reason to believe that the’ 
Scaphopota arose from a stem common to them and the lowest Gastero- 
pods, which are central unspecialised forms. The Cephalopoda are 
the most highly specialised of all the Mollusca, and in their existing 
forms at least not nearly related to the other classes. 
Class I. GASTEROPODA 
Molluscs with a usually well-developed head region with 
tentacles and odontophore. The foot is usually a flat median 
sole on which the animal creeps; it ts often divided into pro-, 
meso-, and meta-podium. Most are unsymmetrical, but there 
ts a primitive bilateral symmetry in. Isopleura and a secondary 
superficial bilateral symmetry in some pelagic forms such as 
Fleteropods. The manile or covering of the visceral sac usually 
Jorms a well-marked fold or flap where the visceral sac joins 
the head and foot, and thus encloses a mantle cavity. In most 
cases the shell ts a single piece; in Chitons there are eight 
pieces; in many cases the shell ts rudimentary or absent. 
There is usually a trochosphere and veliger larva, except in 
terrestrial forms. 
Sub-class I. GasTEROPODA ISOPLEURA 
The [sopleura are marine Gasteropods more or less elongated 
in form, with bilateral symmetry. The symmetry ts not only 
seen in the form of the body, but in the numerous ctenidia, the 
paired nephridia, auricles, and genital ducts. The shell con- 
sists of eight pieces. The mouth ts anterior ; the anal and” 
nephridial apertures are posterior. The mantle, which bears 
cuticular spicules, covers at least a great part of the body. 
The nervous system consists of a cerebral commissure and 
two paired longitudinal cords (pedal and visceral), with 
ganglionic cells but at most very slightly developed ganglia, 
which run the whole length of the body. Of these paired cords 
the pedals are connected by numerous cross-commissures, and 
the viscerals or pallials are united posteriorly by a commissure 
above the rectum. The bilateral symmetry ts shown internally, 
27 
