2o8 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. Pneumodermon has 

 a pair of appendages which bear suckers similar in structure to 

 those found in the Cephalopoda. 



Order 2. Pulmonata. 



Characteeistics. — This second order of the Euthyneura has 

 possibly hecome 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 ctenidium 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 respiratory 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 Limnaea, 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. Onchidium is 

 remarkable for possessing, in addition to the normal pair of 

 ceplialic eyes, a number of dorsal eyes scattered over the 

 integument. These latter, like the eyes in the mantle of 

 Pecten and Spondylus, are not constructed on the usual 



