114 



RELATIONS OF MAN WITH EXTERNAL NATURE. 



In the highest forms, however, there is no true heart or central circulating 

 organ, and the chief nervous ganglion sends oft* nervous cords scattered through 

 the body ; but there is still a central alimentary canal, distinct from and lying 

 in a perfect perivisceral cavity. 



Fig. 45. V. Transverse section of Vertebrate type or plan, r, the same inverted. M. Transverse section 

 of Molluscous type. A. ditto Annulose. Md. ditto Molluscoid. Ad. ditto Annuloid. C. Longitudinal 

 section of Coelentei ate plan, a, alimentary canal; c, body-cavity. P. Diagrams of Infusorial, Amoebous, and 

 Gregarinidous I'rotozoic plan, highly magnified. In the upper six of these diagrams, the alimentary canal 

 is shaded with cross lines; in them, and in the plan C, the nervous cords are left as open rings; the heart 

 or circulator}' vessel, when present, is represented black. 



The sub typical position of these last two -forms, in regard to the Mollusca 

 and Annulosa, is obvious ; of these plans, or types, they are simplifications, 

 or, as in the radiated forms of the Echinodermata, modifications in points of 

 detail. 



The Ccelenterate type is altogether different and lower. A transverse section 

 will no longer reveal its plan. A longitudinal section, C, shows the com- 

 plete absence of a perivisceral cavity, for the alimentary canal, a, now ends 

 in, or is part of, the general cavity of the body, c ; there is no heart or central 

 circulating organ ; and the nervous centre, when such can be detected, is 

 placed at the end of the animal opposite to the oral aperture. 



Lastly, the Protozoic type is still more simple. A section of their bodies no 

 longer indicates even a distinct body-cavity. A diagram of the highest, or 

 Infusorial form, shows a unicellular bi-nucleated sarcodous mass, with a firm 

 exterior bearing cilia, a short gullet ending abruptly in the mass, and one or 

 more internal contractive vesicles ; but no nervous or circulatory apparatus. 

 The Amoabous form has not even a short gullet ; but the soft mass, now with- 

 out a distinct envelope, and changeable in its shape, has a single or double 

 nucleus, and sometimes also a contractile vesicle ; aggregates of such masses 



