112 GENERAL STRUCTURE OF MOLLUSCA. 



rently imbibed through, the whole surface of their bodies 

 from the juices in the midst of which they live ; whilst, on 

 the other hand, their reproductive apparatus is enormously 

 developed, the multiplied segments of the Tape-worm (for 

 example) containing this alone, and the head (as it is com- 

 monly termed, though really the body) being able to repro- 

 duce these to an indefinite extent after they have been 

 thrown off. The group of ROTIFERA, or Wheel-Animalcules, 

 which is one of great interest to the Microscopist, also belongs 

 to this lower section of the Articulated sub-kingdom. 



106. The general character of the animals composing the 

 group or division MOLLUSCA, is, in many respects, the very 

 opposite of that which prevails in the Articulated animals. 

 The body is soft (whence the name of the group is derived), 

 neither possessing an internal skeleton, nor any proper ex- 

 ternal skeleton. In some of the most characteristic specimens 

 of the group, such as the Slug, there is no hard frame- work 

 or skeleton whatever, the body being alike destitute of 

 support and protection. In most Mollusks, however, the 

 body has the power of forming a shelly covering, which serves 

 for its protection ; but this does not give any assistance in its 

 movements by affording fixed points for the attachment of the 

 muscles ; in fact, when the animal puts itself in motion, it is 

 obliged to make its locomotive organs project beyond the 

 shell. We must not regard the shell as an essential part of 

 the Molluscous animal ; because there are many tribes entirely 

 destitute of it ; and also because some of the Articulata have the 

 power of forming a shell ( 102), which bears a close resem- 

 blance to that produced by the animals of this group. Not un- 



Fig. 54. TESTACEILA. 



frequently we see that, of two animals whose general structure 

 is almost exactly the same, as that of the Snail and Slug, 



