CLASSES OF THE MOLLUSC A. 3 



Protozoa. In the same way we can pass from the highest of the 

 Aunulosa to the Protozoa. But we cannot find any continuous 

 succession of adult forms which will connect the Annulosa with 

 the Mollusca, or the Mollusca with the Yertebrata. 



Much use is made of the terms high and low in speaking of 

 animals ; and it is important to bear in mind that they are by 

 no means intended to imply that there is any difference in the 

 degree of perfection, or that one animal is less fitted to subserve 

 the purposes of life than another. By an animal of a low 

 organisation is simply meant one in which all the functions 

 of life are carried on by means of a few organs. The greater 

 the number of organs that are set apart to perform special 

 functions the higher is the animal said to be. 



The evidence afforded by geological researches seems to show 

 that the leading types of animal structure have existed from a 

 comparatively early period in the history of the globe ; and that 

 all forms which have left any indications of their existence 

 belong to one or other of these types. The oldest fossils known 

 at the present time belong to the Protozoa ; but next to them 

 come the Mollusca. 



By adding to the living population of the world, those forms 

 which peopled it in times long past, we may arrive at some 

 dim conception of the great scheme of the animal kingdom. 

 And if at present we see not the limits of the temple of nature, 

 nor fully comprehend its design, — at least we can feel sure that 

 there is a boundary to this present order of things ; and that 

 there has been a plan, such as we, from our mental constitu- 

 tion, are able to appreciate, and to study with ever-increasing 

 admiration. 



Classes of the Mollusca. 



This sub-kingdom consists of two gioat groups, viz., the 

 mollusca proper and the moUuscoida. The mollusca are animals 

 with soft bodies, enveloped in a muscular skin, and usually 

 protected by a univalve or bivalve shell. That part of their 

 integument which contains the viscera and secretes the shell, is 

 termed the mantle ; in the univalves it takes the form of a sac, 

 with an opening in front, from which the head and locomotive 

 organs project : in the bivalves it is divided into two lobes. 



The univalve mollusca are encephalous, or fui-nished with a 

 distinct head ; they have eyes and tentacula, and the mouth is 

 armed either with jaws or with tooth straps.* Cuvier has 



* One of the drawbacks to the study of mollusca is the prevalence of such terms aa 

 Jaws, arms, feet, &o. The reader must not suppose that the parts so designated are 



b2 



