VII MOLLUSGA—THE FOOT AND ITS GLANDS 111 



detached.^ The parapodia are large, iin- or -wiiig-like, and anteriorly inserted on each 

 side of the median portion of the foot ; they unite in front of and above the month. 



The Gymnosomata (Fig. 16, p. 11) are to be derived from the Aphjsiidce, in 

 whicli the paraj^odia are not exactly lateral extensions of the sole of the foot, but 

 arise somewhat above the edge of the sole on each side. This may be explained by 

 supposing that they are fused for a certain distance from their bases with the lateral 

 wall of the body. In the Gymnosomata, also, the foot is distinctly sejjarated from 

 the two lateral fins or parapodia. The mesopodium and the fins lie anteriorly on 

 the ventral side of the body, behind the head. 



The foot itself, which is distinct from the head, consists of three })arts — a j^air of 

 anterior lobes, which converge anteriorly till they unite, and a median posterior lobe 

 dra^\^l out to a point posteriorly. The fins never unite in front of or above the head. 



Pedal glands of the Gastropoda. — Many Gastropods, and especially 

 most Prosohranchia and Puhnonata, possess, besides the various unicellular 

 glands scattered over the upper and lower sides of the foot, larger multi- 

 cellular localised pedal glands. These belong to two morphologically 

 distinct groups. 



1. In the Frosobranchia an anterior pedal gland opens at the 

 anterior edge of the foot. In those forms in which this anterior edge 

 is divided into an upper and a lower lip, this " labial gland " opens 

 between the lips. In the Pulmonata it opens externally between the 

 head and the foot. It consists of an epithelial tube of varying length, 

 not infrequently as long as the foot itself ; this tube runs backward 

 in the median line mostly through the base of the foot ; less frequently 

 it lies upon this base, projecting into the body cavity. 



This tube serves both as reservoir and duct for the numerous 

 unicellular mucous glands which lie in the surrounding tissue of the foot 

 and open on its walls. It secretes mucus, though it has been incorrectly 

 described as an olfactory organ. It undergoes considerable modifica- 

 tions with regard to its size, the form of its lumen, and the number and 

 arrangement of its glandular cells. 



2. Among the Prosobranchia, opening on the sole of the foot, there 

 is commonly found an unpaired gland. Its outer slit-like aperture is 

 median, and lies behind the anterior edge of the foot. It leads into a 

 cavity in the foot which serves as a reservoir ; the epithelial wall of 

 this cavity projects in the form of folds into its lumen. As in the 

 former case, unicellular glands pour their secretions into it through 

 ducts which pass between the epithelial cells. This sole gland in the 

 Prosohranchia has rightly been considered homologous with the byssus 

 gland of the Lamellihranchia. It is developed in varying degrees, and 

 not infrequently is altogether wanting. Its slimy secretion forms 

 threads by means of which many Prosobranchia attach themselves to 

 objects in the water. Some terrestrial Pulmonata also lower themselves 

 from a height (from plants) by means of the tough threads which they 

 secrete. 



^ With regard to the derivation of the Thecosomata from the Cephalaspidcc, which, like 

 other Opisthobranchia, have as a rule no operculum, it must be noted that A ctceon, which 

 is iu many respects a primitive Ce2}halaspid genus, possesses an operculum. 



