86 THE GASTROPODA 
and without a spiral coil. At the same time there is a return to a 
secondary external symmetry. This phenomenon may be seen in 
the Titiscaniidae, Pferotrachea (Fig. 148), Runcina, Phyllaplysia, the 
gymnosomatous “ Pteropods” (Fig. 84), the Cymbuliidae (Fig. 151), 
Pleurobranchaea (Fig. 157), the Nudibranchs (Figs. 160, 161, ete.), 
the Philomycidae, the Oncidiidae (Fig. 59), and the Vaginulidae 
(Fig. 179). In these cases, excepting Cenia and Runcina, the shell 
exists only during development, and falls off at the close of larval 
life. As a rule, the pallial cavity is reduced at the same time 
(Pterotrachea, Pleurobranchaea, Gastropteron), or it may disappear 
together with the ctenidium, and the external surface of the dorsal 
visceral envelope may give rise to various appendages such as the 
cerata or dorsal “ branchial” papillae of Nudibranchs (Fig. 160, A, c) 
and the terminal branchia of the Gymnosomata (Fig. 84, VII). In 
one instance in which the larval shell is caducous a second per- 
sistent shell is formed, covered by the mantle: such is the case in 
Lamellaria, whose primary shell is covered with spines, and was for- 
merly believed to belong to another animal to which the name of 
Echinospira was given. 
In parasitic Gastropods the naked condition of the adult is the 
result of an essentially similar process, but the shell is covered over 
by a cephalic expansion known as the “ pseudopallium ” (Fig. 20, ps). 
Finally, the nudity of certain “ Heteropods” (Pterotracheidae) is 
due to the progressive reduction of the visceral sac and the dis- 
appearance of the mantle. In many Gastropods that are naked in 
the adult condition calcareous spicules of some size are developed 
in the sub-epithelial conjunctive tissue of the mantle, ¢.g. in the 
Pleurobranchidae and in Doridomorpha, the Hedylidae among the 
Nudibranchs. In the Cymbulidae (Fig. 151, II) the sub-epithelial 
connective tissue gives rise to a pseudo-conch. 
II. ANATOMY. 
1. The Alimentary Canal.—This comprises, in the various forms 
of Gastropods, a buccal cavity connected by the oesophagus with a 
stomachal cavity, and an intestine properly so-called, the last-named 
being tolerably long and coiled. The buccal cavity and the 
oesophagus are of ectodermic origin, and taken together form 
the fore gut. The buccal cavity normally opens at the extremity 
of the head, which generally has the form of a cylindrical snout 
slightly inflected towards the ventral surface (Fig. 130, VI). In 
many cases, however, the opening of the buccal cavity is carried 
backward by the development of an invagination of the pre-oral 
integuments, and thus an apparent mouth is formed which is not 
morphologically equivalent to the true mouth, the latter being 
carried to the anterior extremity only by an evagination of the 
