218 



THE SEA SHORE 



be easily distinguished from similar species of another genus by 

 the very pointed umbones, and the coarse and strong fibrous 

 byssus by which it clings to any solid object. It is found most 

 abundantly on muddy coasts, and on mud banks in the estuaries 

 of rivers, generally in such situations as are uncovered at low tide. 

 The fry abound just below low-water level, and grow so rapidly 

 that they reach their full size in a single year. 



It is well known that a diet of mussels occasionally produces 

 very unpleasant and even dangerous symptoms in the consumer, and 

 this result has been attributed to the action of a particular organ 



of the animal which has 

 not been carefully removed 

 before eating. This, how- 

 ever, is not the case, as 

 proved by the fact that 

 the eating of these edibles 

 is usually perfectly safe 

 when no such precautions 

 have been taken. It is 

 highly probable that the 

 deleterious character re- 

 ferred to is due to a disease 

 which sometimes attacks 

 the mussels themselves, 

 but the exact nature of this 

 has not been thoroughly 

 made out. 



There is another genus 

 (Modiola) containing se- 

 veral species commonly 

 known as Horse Mussels, 



and these may be distinguished from Mytilus by their habit of 

 burrowing, or of constructing a nest by spinning together various 

 fragments. The shell, also, is more oblong in form, and much 

 swollen near the anterior end ; and the umbones are not so pointed. 

 The epidermis covering the shell is of fibrous structure, and often 

 extends beyond the edges of the valves in the form of a fringe. 



Several species of Horse Mussels inhabit our shores, from low- 

 water mark to a depth of fifty fathoms, but none of them is used 

 for food. The commonest species is Modiola modiolus, which has 

 a particularly strong byssus, and its fibres generally bind together 



FIG. 150. 1. Modiola modiolus. 

 2. Modiola tulipa. 3. Crenella discors 



