292 MOLLUSCA. 



a tube runs off to the anterior and posterior end of the body. 

 The blood is colourless or pale yellow or pale green, and consists 

 of amoeboid corpuscles only. The nutritive substances derived 

 from the food in the digestive tube pass through the walls of the 

 tube and mix with the blood in the neighbouring vessels. There 

 are no lymphatics as in vertebrate animals. A large muscular 

 area used for locomotion is found in most groups, called the 

 " foot " (iig. 169, /). This structure is much modified according 



Fig. 160. —Edible Bumj^ (Helix pomatia). 

 T, tentacles ; E, eyes ; a, sm.^^ Icntacles ; /, foot. (Brit. Mus. Guide.) 



to its function : in some it is vised for boring, in others for 

 crawling, and yet others use it for leaping. There are two chief 

 types of shells formed by these invertebrates — Bivalve (fig. 171) 

 and Univalve (fig. 173) shells : the former consist of two valves 

 united by an area called the hinge-line ; the other may be a 

 simple tube, a spirally coiled tube, or a conical flattened disc. 



Eeproduction in jNlollusca is sexual : both the sexual organs 

 may be in the same individual or in separate individuals. The 

 former (hermaphrodites), nevertheless, reproduce by cross-fertil- 

 isation. The majority of land Mollusca develop direct ; but in 

 many aquatic species there is a larval form. 



In Unio or the Fresh-water INIussel the ovaries and testes — the 

 female and male genital glands — are found in the foot. They 

 are in separate individuals (dicecious). They are large in winter 

 and spring, as the breeding season is then on. The male cells, 

 spermatozoa, are discharged into the water with a milky fluid, and 



