8 FRESHWATER MOLLUSKS. 



But to leave them for the LamellihrancMata, of which I hope the 

 canal will furnish us some specimens. 



Bearing in mind that all mollusks with a double shell are set down 

 as "oysters," "cockles," or "mussels," by the uninitiated, I beg you all 

 to keep a good look out to-day for " mussels" and " cockles." 



Now just a word or two about the way in which these Lamellibran- 

 chiata are grouped. 



When they are in their ordinary condition you will see them in the 

 water with one or more tubes projecting from the partly open shell for 

 inhaling and exhaling the surrounding fluid; but if they are disturbed 

 or alarmed in any Way the two shells are drawn close together. Now 

 you will ask, how do they close their shells ? In a very simple manner, 

 by having a muscle attached to each shell, which they have the power 

 of expanding or contracting at their will, very much like those India 

 rubber springs with which doors are sometimes kept closed. 



Now some of these LamellihrancMata have one of these muscles 

 attached to each shell, and form a class called the Monomyaria, or one- 

 muscled Lamellibranchiates. Of these the oyster is a common example, 

 and the next time you have an oyster shell in your hand, you will see 

 on it a scar where the muscle was attached. Of this group we find 

 only one in freshwater in our country ; and I fear we must not expect 

 to find it to-day, although it is far from improbable that there are 

 some to be met with in this canal. 



Others of the Lamellibranchiata have two of these muscles attached 

 to each shell, and are called Dimyaria, or two-muscled La/mellibran- 

 chiates; these are the "cockles" and "mussels." In them you will see 

 two scars on each shell. 



1st The Anodomta cygnea, or Swan Mussel, of which we may, I hope, 

 be fortunate enough to obtain specimens three or four inches in length, 

 and which have been known to attain as great a measurement as nine 

 inches. 



2nd. Unio, a smaller and rounder mussel than the preceding, of 

 which there are three species, that I shall be happy to name if any 

 gentleman finds specimens. 



3rd. Cyclas, a kind of freshwater cockle, of which there are five 

 species, all small, the largest barely an inch aci'oss. 



4th. Pisidium, another cockle, of which there are seven species, the 

 largest no bigger than a pea. 



These are all the Lamellibranchiates to be found in English fresh water. 



Wo must now turn to the Encepliala, or mollusks with heads. 



These arc divided into fa) Pteropocla, creatures that swim by two 

 wing-like muscular expansions extended outwards from the sides of the 

 head. There are only three or four modern genera, all found in salt 

 water. 



(b) The Cephalopoda, creatures having their feet or organs of motion 

 attached to their head, so that literally it is a question whether they 

 stand on their head or their heels, aa the expression is. These we 



