TYPES AND LAWS OF FORM. 113 



relations, mysterious though they be, to the present, to a past and a 

 future, to the finite, and to the Infinite. 



Types, Laws of Form, Homologies, Analogies, Unity in Variety, 

 G-enetie Relations. 



The zoological relations of man with the entire animal kingdom, 

 are necessarily associated with anatomical and physiological resem- 

 blances and differences more or less marked in special cases. The 

 determination of these is the proper object of comparative anatomy 

 and physiology. The former science has been cultivated so far as to 

 lead to the discrimination of certain general plans or types of form 

 observable in the animal series, which are indicated in the several sub- 

 kingdoms. At least, there can be little doubt as to the apparent dis- 

 tinctness of the vertebrate, molluscous, annulose, coelenterate, and pro- 

 tozoic types; though it is possible that the molluscoid and annuloid 

 groups are subtypical, and attached respectively to the molluscous and 

 annulose types. The ideal plans of these types of course involve every 

 leading or essential feature in their structure; but one very simple 

 view of them, is that expressed by a comparison of transverse sections 

 through the body in each case, as is shown in the following diagrams, 

 Fig. 45. 



Thus, a transverse diagrammatic section of the body of a Vertebrate animal, 

 V, shows two chambers, or perivisceral cavities, an upper smaller one, and a 

 lower larger one, separated from each other by the more solid or axial part of 

 the vertebral column, occupying the position of the so-called dorsal cord of the 

 embryo. In the upper tubular chamber, the neural cavity, lies the section of 

 the great nervous axis or centre ; in the lower chamber, or hcemal cavity, are 

 lodged the double or laterally symmetrical sympathetic nerves, above, the 

 alimentary canal in the middle, and the heart, or central organ of the blood 

 system, below. 



A transverse section of a Molluscous animal, M, shows but a single body- 

 cavity or perivisceral cavity, in which the heart is placed above, the alimen- 

 tary canal in the middle, and the chief portions of the nervous system, i. e. 

 the double laterally symmetrical pedal and parieto-splanchnic ganglia, at the 

 lower part. 



A similar section of an Annulose animal, A, presents also a single perivis- 

 ceral cavity, having as in the Mollusc, the alimentary canal in the middle, the 

 double ganglionated nervous cords below, and the elongated dorsal circulating 

 vessel above. 



On comparing the Molluscous and Annulose types, it appears that the longi- 

 tudinal segmentation of the latter is the chief difference, the typical plan 

 being otherwise the same. On comparing these two with the Vertebrate type 

 (which can best be done by supposing the latter to be inverted, so that its ab- 

 dominal surface is turned upwards, as at v), it will be seen that their single 

 perivisceral cavity, with its contents, appears to represent the haemal cavity 

 of the Vertebrate animal and its contents ; and that the vertebral column, or 

 dorsal cord, the neural cavity, and its great nervous axis, the cerebro-spinal 

 centre, are altogether superadded parts in the Vertebrate type. 



A transverse section through a Molluscoid animal, Md, shows also a single 

 perivisceral cavity, having the alimentary canal in the middle, the single or 

 chief ganglion on the side next the locomotive organs, and the rudimentary 

 heart on the opposite side ; but these sides are no longer obviously under and 

 upper, as in the Mollusca. 



No single diagram will represent the diverse plans of the Annuloid animals. 



