122 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Many shell-fish, e.g., the salt-water Mussel, are attached during life to 

 surrounding objects by threads forming the byssus. 



The embryo now passes out of the gill of the parent, being 

 ejected from a small opening bounded by the coalesced mantle- 

 lobes, and situated some distance above the exhalent aperture. 

 The young mussel is very unlike the adult. It was formerly 

 thought to be a distinct mollusc, and named Glochidium. This, 

 however, was afterwards found to be only a larva i.e., a free- 

 living embryo, very dissimilar to the mature form, and undergoing 

 a series of changes known collectively as a metamorphosis before 

 becoming adult. 



The Glochidium has special larval sense organs, by which it 

 is enabled to detect the presence of fish, and it swims freely by 

 napping the valves of the shell. The long byssus trails behind, 

 and if it comes into contact with the skin of a fish adheres to it, 

 and the sharp spines on the valves then effect a firmer hold. In 

 this position the larva is covered over by outgrowths from the 

 skin of the fish, and undergoes metamorphosis. The byssus and 

 larval sense organs are lost, while the foot, gills, and internal 

 organs are gradually developed. The host is then left. 



This peculiar parasitic habit of the larva prevents it from being 

 washed down to sea by the current (see p. 107). 



12. HELIX (The Snail). 



The two species of Helix most available in Britain for dissec- 

 tion are Helix aspersa, the common snail, and Helix pomatia, the 

 Eoman snail. H. pomatia is characterized by its much larger 

 size, and lighter colour. In the essential features of its organ- 

 ization, however, it differs but little from H. aspersa, which, as 

 the commoner kind, will be described here. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The most striking feature is the pos- 

 session of a shell, into which the animal can entirely withdraw 



