202 



ZOOLOGY. 



g 



portion of this system occupies the dorsal aspect, and another 

 portion the ventral aspect, of tin- digestive tube; but these 

 ganglions do not form a long median chain, as in the pre- 

 ceding division. 



They have no skeleton internally 

 or externally; their body is soft. 

 and their skin constitutes a flexible 

 and contractile envelope or mantle; 

 it is often covered with horny or 

 calcareous plates, called shells (Fig. 

 143), and this is sometimes de- 

 veloped in its interior. In this 

 primary division the organs of the 

 senses are almost always very incom- 

 plete; there seems to be no special 

 organism for smell, and in a great 

 number the eyes are wanting ; they 

 have hardly ever limbs for loco- 

 motion ; and finally, the blood is 

 white, as in most of the annelides, 

 but the circulation is often much more complete. 



377. Zoophytes, Finally, in the fourth and last primary 

 division, the different parts of the body, in place of being 

 grouped symmetrically with reference to a median plane, 

 tend rather to arrange themselves around a central point or 



P 



Fig. 142." 



Fig. 143. Linnaeus of Stagnant Waters. 



* Ideal section of the body of a cephalopodous mollusc : t, arms or ten- 

 tacula surrounding the head; 6, the mouth ; t, the alimentary canal ; a, the 

 anus ; f, the liver ; c and g, nervous ganglions; p, branchiae ; *, the heart ; 

 a, reproductive apparatus ; t, ink bag or vesicle ; y, the eyes. 



