486 THE INVERTEBRATA 



occupies most of the visceral hump. The " liver", though apparently 

 solid, is composed of a number of tubes and the end portion {alveolus) 

 of each tube is glandular ; the rest is ciliated and serves to introduce 

 small fragments of food into the active alveoli. The alveoli contain cells 

 of three kinds, secretory, resorptive and lime-containing (Fig. 358 A). 

 The secretory cells produce the brown fluid found in the crop ; this 

 contains a ferment which dissolves the cellulose of plant cell walls and 

 liberates the protoplasmic contents, no portion of which is digested 

 in the crop or stomach. But these contents in the form of small 

 granules are actually introduced into the alveoli of the liver and there 

 taken up and digested by the resorptive cells which possess an intra- 

 cellular proteolytic ferment. A combination of extra- and intracellular 

 digestion is highly characteristic of Mollusca, but in the possession 

 of a cellulose-dissolving ferment Helix stands almost alone in the 

 Animal Kingdom, and may be indeed said to be physiologically 

 adapted to a plant diet. The intestine runs from the stomach, within 

 the liver, and then as the rectum in the roof of the mantle cavity. 



The reproductive organs are extremely complicated (Fig. 361VA), 

 but a function has been assigned to each part of what appears to the 

 elementary student as an unmeaning tangle of tubes. Eggs and sperm 

 are produced in the same follicle of the ovotestis, a small white gland 

 in the apex of the visceral hump. But while ripe sperm is found 

 throughout a large part of the year, mature eggs only occur for a very 

 short space indeed. Both eggs and sperm pass from the ovotestis to 

 the albumen gland through the herinaphrodite duct, the terminal 

 portion of which is a pouch (receptaculum semifiis) where sperm 

 is stored and fertilization is said to occur. After fertilization, the eggs 

 enveloped in albumen from the gland enter the rather voluminous 

 female duct, which runs almost straight to the exterior. They then 

 receive a calcareous shell secreted by the epithelium of the duct. The 

 terminal portion of the duct is the thick-walled muscular vagina, into 

 which open the mucous glands, the dart sac and the spermathecal duct . 

 The sperm, on the other hand, passes down a male duct which is at 

 first only partly separate from the female duct, the cavity of both ducts 

 being in communication until the male duct leaves the company of 

 the female duct altogether, slips under a muscle, and joins the penis 

 at its junction with the slender flagellum . In this latter the spermatozoa 

 are compacted together to form spermatophores. The penis is muscular 

 and has a special retractor penis muscle also attached to it. Both 

 vagina and penis open into a common genital atrium, with an opening 

 to the exterior far forward on the right side. 



Cross-fertilization is the rule in nearly all species of Helix but cases 

 of self-fertilization have been known. Usually, however, there is re- 

 ciprocal fertilization, preceded by a remarkable preparatory event in 



