fueshwatee mollusks. 



FIELD DAY, MAY and. 



The members met on the Lees, and proceeded to the canal 

 at Seabrooke. The Eev. E. Langdon read the following 

 paper on 



FRESHWATER MOLLUSKS: 



The tmllusca, or mollusks, form one of the most numerous specifically 

 and individually, and the most widely distributed, both in time and 

 space, of all the great divisions of the animal kingdom. Most widely 

 distributed in time, for geologically we find their remains embedded 

 in all rocks of sedimentary origin, from the very oldest to the most 

 recent; the most widely distributed in space, for iu every quarter of 

 ths globe where life can be supported the moUusca in some form are 

 met with. Called mollusks from the softness of their bodies, they have 

 no articulated skeleton nor vetebral canal. Their nervous system is 

 not united as in the vertebrata by a spinal cord, but scattered about in 

 nervous masses, disposed in various parts of the body ; the principal 

 one, or brain, if we may so call it, forming a nervous collar or ring 

 round the gullet. A large number of them have no head or brain, as 

 having no need of nerves for the transmission of the impressions 

 received by organs of special sense. In them, the inlet foi' food is 

 simply an opening or beginning of the alimentary canal, without jaws, 

 tongue, or mouth, properly so called. All the remaining mollusks are 

 provided with a head, which generally support feelers, or tentacles as 

 they are called, eyes, and a mouth armed with jaws. So that we at 

 once get a good division of them into two classes, both of which I 

 hope I shall be able to shew you to-day : 



AcEPHAr.A, or mollusks without heads. 



ExcjiPHALA, or mollusks with heads. 



The headless mollusks all live in the water, and are divided intu 

 three further classes : 



Tunicata, Brachinpoda, and Lamellihranchiata, 



Of these, the first two I will pass over, as we shall be unable to 

 obtain any practical knowledge of them to-day, merely pausing to 

 make one remark about the Brachio^JOtJji. They are among the most 

 abundant of the molluscous remains that the geologist finds in the 

 early deposits, and very abundant in all up to the time of the chalk, 

 after which thoy become less frequent ; but the point to which. I wish 

 to call especial attention is that the geologist knows somewhere about 

 3,000 species, whereas in our own times only 13 species are known, and 

 those very difficult to be procm-ed. 



