BIVALVES AND CUTTLES. 271 



animal does fix itself firmly to rocks, and it is of interest 

 to notice how it accomplishes this. Of the two adductor 

 or closing muscles which most Bivalves possess, one is here 

 very rudimentary, while the other is large and conspicuous. 

 From this large muscle a slip arises which passes through 

 the hole in the lower valve of the shell, and is attached 

 to the rock. Its end is furnished with a curious limy disc 

 or operculum, formed by an aggregation of many little 

 plates. In the living animal the shell, as usual, gapes to 

 allow for the entrance of the necessary currents of water, 

 and there is also, owing to the relaxed condition of the 

 attaching muscle, a space between the lower valve and the 

 rock. When the animal is alarmed the muscle contracts 

 suddenly, the result being not only to close the valves, 

 but also to drag the lower valve close against the rock. 

 You will appreciate the meaning of this best by trying to 

 peel the animals off the rock with your fingers, after they 

 have been thoroughly alarmed. The method of attachment 

 is very interesting, and offers some curious problems in 

 origins to those speculatively inclined. Why should the 

 saddle-oyster have given up its original method of attaching 

 itself by a byssus? And if its peculiar method is advan- 

 tageous, why should other sedentary forms have not adopted 

 it ? There are many similar questions which one may ask, 

 though I do not know if they can be answered at present. 



Saddle-oysters are very abundant on the shore, but on 

 the East are usually of small size, the shells being often 

 under half an inch in diameter. From their habit of closely 

 accommodating their shells to the irregularities of the rock 

 surface, they are very apt to be overlooked by careless 

 observers. 



We come next to the mussels, familiar forms, unfor- 

 tunately burdened with a plethora of names. We shall 

 describe three species, placed in as many different genera, 

 viz. the edible mussel (Mytilus edulis), the horse -mussel 

 (Modiola modiolus), and the marbled Crenella (Crenella 

 marmorata). All three are nearly related, and are some- 

 times placed in the same genus, or given other generic 

 names. All are common, and may easily be found in the 

 living state. 



The edible mussel is easily recognised, and has already 



