MOLLDSCA. 183 



SUB-KINGDOM V. MOLLUSCA. 



CHAPTEH XVIII. 



Sub-Kingdom Mollusca. 



The Mollusca (Lat. mollis, soft), as implied by their scientific name, 

 are mostly soft-bodied animals ; but their popular name of " shell- 

 fish '' expresses the fact that their soft body is usually protected by 

 an external skeleton or "shell." All the Mulhusca are furnished 

 with a distinct alimentary canal, which is completely shut off from 

 the general cavity of the body. There is sometimes no distinct 

 blood-circulatory apparatus ; but when there is, its central portion 

 (i.e., the heai-t) is placed upon the dorsal aspect of the body. The 

 chief peculiarity, however, of the Mollusca is found in the nature of 

 the nervous system. In the lower forms (fig. 132, 2, d), the nervous 

 system consists essentially of a single ganglionic mass, giving off 

 filaments in various directions. In the higher Mollusca (fig. 129), 

 the nervous system ci;nsists of thi'ee scattered pairs of ganglia, united 

 to one another by nervous cords. One of these pairs is placed above 

 the gullet or (Ksojjhagus, and is known as the " supra-cesophageal " or 

 " cerebral " ganglia. A second pair supplies nerves to the great loco- 

 motive organ of most Molluscs, the "foot," and is therefore called the 

 " pedal " ganglia. The third pair is known by the cumbrous name 

 of the " parieto - splanchnic " ganglia, because it supplies nervous 

 filaments to the walls (parietes) of the body, and also to the internal 

 organs (splaiichna). In all the higher Molluscs it is this .scattered 

 condition of the nervous masses which distinguishes them so sharply 

 from all other animals. Distinct respiratory organs may or may not 

 be present, and they may be adapted for breathing air directly or 

 through the medium of water. All the higher Mollusca are simple 

 animals, and perpetuate their kind by means of the sexes ; but many 

 of the lower forms have the power of producing colonies by con- 

 tinuous gemmation, much as we have formerly seen in the Hydroid 

 Zoophytes. 



The digestive system in all the Mollii-ica consists of a mouth, 



