114 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



tubules, closely surrounds the stomach, into which it opens by 

 several ducts. 



The alimentary canal is lined throughout by simple columnar 

 epithelium, which is partly ciliated, partly glandular. External 

 to this are muscular layers. The tubules of which the digestive 

 gland is made up are lined by glandular epithelium, the cells of 

 which are cuboidal with brown granular contents. By the succes- 

 sive unions of these tubules the ducts are ultimately formed. 



The ciliated epithelium, lining the mantle-lobes and covering 

 the gills and labial palps, sets up currents in the mantle-cavity, 

 and causes water to enter the branchial chamber by the inhalent 

 orifice. Some of this is conducted along the groove between each 

 pair of labial palps to the mouth, into which the small organisms 

 it contains are carried. These constitute the food, and by the 

 contraction of the muscular walls of the alimentary canal gradu- 

 ally pass backwards within it. The fluid secreted by the digestive 

 gland contains ferments by the action of which fats are emul- 

 sified, while starch and proteid are respectively converted into 

 grape-sugar and peptone. The length of the intestine increases 

 the surface for absorption, and this is augmented by the typhlosole. 

 The digested parts of the food diffuse into the blood-vessels of 

 the intestinal walls, and the refuse is ejected at the anus, being 

 afterwards carried out of the cloacal chamber by the currents 

 which flow from the exhalent aperture as a result of ciliary 

 action. 



In specimens examined during autumn a transparent elastic rod, 

 the crystalline style, will be found in the stomach. It is of albu- 

 minous nature and probably serves as a store of nutriment, for it 

 is gradually used up during the winter. Some material of similar 

 kind, only less compacted, is found in the intestine at the same 

 period. 



4. Circulatory Organs (Figs. 32 and 34). A blood system 

 alone is present, containing colourless blood in which are sus- 

 pended numerous nucleated amoeboid corpuscles devoid of colour. 

 This system is constituted by heart, arteries, veins, and irregular 

 blood-spaces. The heart, which is contained within a thin-walled 

 pericardial cavity situated on the dorsal side of the body, consists 

 of a central oval muscular ventricle, into which a thin-walled 

 auricle opens on each side. The auricles are funnel-shaped, with 

 the narrow end attached to the ventricle, the broad end to the 



