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, Pterotrachea (Fig. 1 43), Euncina, Phyllaplysia, the 

 gymnosomatous " Pteropods" (Fig. 84), the Cymbuliidae (Fig. 151), 

 Pleurobranchaea (Fig. 157), the Nudibranchs (Figs. 160, 161, etc.), 

 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, Gastropterori), 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, e.g. in the 

 Pleurobranchidae and in Doridomorpha, the Hedylidae among the 

 Nudibranchs. In the Cymbuliidae (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 



