208 ZOOLOGY 
modified into fins, which in the Thecosomata embrace the 
head. The anterior portion of the digestive tract is evaginable 
in Clio, and bears a number of unicellular glands opening upon 
cones. The secretion of these glands is adhesive, and they 
serve as organs for the capture of prey. Pnewmodermon has 
a pair of appendages which bear suckers similar in structure to 
those found in the Cephalopoda. 
Order 2. PULMONATA. 
CHARACTERISTICS.— This second order of the Euthyneura has 
possibly become modified from a palliate Opisthobranch 
ancestor, in correspondence with the altered conditions of life 
involved in the change from an aquatic to a terrestrial 
habitat. The etenidium has atrophied, and the edge of 
the mantle has fused with the body-wall, leaving only one 
small opening which leads from the pallial chamber to the 
exterior—this is the resprratory pore. The walls of the pallial 
chamber are very vascular, and they function as lungs. 
An operculum is never present. 
This order includes the land-snails and slugs. As a rule, 
its members live on land, and they all breathe air when adult, 
even those like the pond-snail Zimnaea, which lives in water, 
but whose mantle chamber contains air. The mantle cavity of 
the young freshwater Pulmonates is stated at first to contain 
water, this is afterwards replaced by air. The great extent to 
which the blood-vessels and capillaries are developed in the 
Pulmonata is possibly connected with this habit. 
No true operculum is found in any Pulmonate, but many 
secrete a temporary lid to their shell when they withdraw into 
it for the winter. This temporary operculum is called a 
hibernaculum. In the slugs the shell may be small and even 
quite atrophied. 
Onchidium and Peronia are members of a family which 
inhabit the sea-shore and brackish marshes. Onchidiwm is 
remarkable for possessing, in addition to the normal pair of 
cephalic eyes, a number of dorsal eyes scattered over the 
integument. These latter, like the eyes in the mantle of 
Pecien and Spondylus, are not constructed on the usual 
