48 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



much higher than is met with among the Articulata ; and, in proportion to 

 that of the organs of Locomotion, it is much greater than will be elsewhere 

 observed throughout the Animal kingdom. The justice of this statement will 

 be made evident by a slight examination of the preceding figure, in which 

 the interior structure of the Jlplysia, showing the general character of that 

 of the group, is displayed. The only distinct set of muscles, possessed by 

 this animal, is that connected with the mouth ; which it is able to push for- 

 wards or to draw back, and which possesses considerable powers of mastica- 

 tion, and is furnished with large salivary glands. The nervous centres (of 

 which more will be said hereafter) are seen to be principally disposed around 

 the ffisophagus. The whole digestive apparatus is observed to be very com- 

 plex and highly developed ; the liver alone occupying a considerable part of 

 the cavity. The heart has distinct muscular walls, and is divided into a 

 separate auricle and ventricle ; and a large respiratory organ is developed for 

 the aeration of the blood. The position of the gills, which are external to 

 the cavity, but which are concealed in part by a fold of the mantle, and in 

 part by the rudimentary shell, is seen at a, Fig. 2. The generative apparatus, 

 also, is highly developed. Yet with all this complex organization, the loco- 

 motive power of the animal is not much greater than that of the Slug; no 

 other means being provided for the purpose than the contractility of the gene- 

 ral envelope, which is greatest in the thickened portion on the under side of 

 the body. 



15. The blood of the Mollusca is white, and the number of corpuscles in 

 it is small. Their temperature is low, being seldom more than one or two 

 degrees above that of the surrounding medium; but many of them are capa- 

 ble of being subjected to extreme variations of heat and cold, without their 

 vitality being thereby destroyed. Their respiration is for the most part aquatic ; 

 and is performed by means of gills, over which a current of water is con- 

 stantly being propelled, by the vibration of the cilia that cover their surface. 

 Many of them are dependent on the same current for their supplies of food ; 

 part of the water so introduced being taken into the stomach ; and a part 

 flowing over the respiratory surface. The higher tribes, however, go in 

 search of their food, and have instruments of mastication for reducing it; but 

 in these, as in the former, the anal orifice of the intestine opens into the 

 passage through which the current that has passed over the respiratory organs 

 finds egress ; so that the faecal matter from the former, Snd the fluid that has 

 served the purpose of the latter, are discharged together. Although very 

 voracious when supplies of food come in their way, most of the Mollusca are 

 capable of fasting for long intervals, where none offer themselves, a fact 

 which is readily explained by that general inertness of their vital processes, 

 which has been stated to be characteristic of the group. 



5. General characters of Articulata. 



16. The members of the sub-kingdom, ARTICULATA, are distinguished, for 

 the most part, by characters which are exactly opposed to those just enume- 

 rated. Their characteristic form is easily defined ; and in no instance is there 

 any wide departure from it. The body is more or less elongated, and presents 

 throughout a most exact bi-lateral symmetry. It is completely inclosed in an 

 integument of greater density than the rest of the structure, which is divided 

 into distinct rings or segments ; these, being held together by a flexible mem- 

 brane, allow considerable freedom of motion, whilst they firmly protect the 

 soft parts, and afford attachment to numerous muscles. It is in the Centipede, 

 and other such animals, that this division into segments is most distinctly and 

 regularly marked. In the lower Articulata, such as the Leech and the Earth- 



