2 MANITAL OF THE MOLLUSC A. 



BucL as Conularia, are ricli in the iDliospliate of lime. Tlie 

 digestiye cavity is completely separated from the walls of tho 

 body. The nervous system consists of three pairs of ganglia, 

 except in the Brachiopoda, and these nervous centres are very 

 much scattered. Hence Professor Owen has proposed the term 

 Heterogaiigliaia for the great grou]^ of MoUusca. The end of 

 the alimentary canal nearest tho mouth is sui rounded by the 

 ganglia which supply the foot and head. 



3. The various tribes of insects, spiders, crabs, starfishes, 

 echinoderms, entozoa, and worms, have no internal skeleton ; 

 but to compensate for it, their outer integument is sufficiently 

 hard to serve at once as a support, a covering, and a defence 

 for the soft parts. This external armature, like the bodies and 

 limbs which it covers, is divided into segments or joints, which 

 well distinguishes the members of this group from the others. 

 Tlie proj)riety of arranging worms with insects will be seen, if 

 it be remembered that even the butterfly and bee commenco 

 existence in a very worm-like form. This division of jointed 

 animals bears the name of the Annulosa. The nervous system 

 consists of ganglia arranged in pairs in the middle line of the 

 body. Prom this equal lateral development of the nervous 

 centres Professor Owen calls the group Homogangliata. Tho 

 nervous system is traversed by the alimentary canal. The 

 radiated animals form a j)art of this sub-kingdom. 



4. The next sub-kingdom comiDrises most of the polypes, 

 such as sea-anemones, the fresh-water hydra, and oorals, in 

 which the general cavity of the body communicates freely with 

 that of the digestive apj)aratus, on which account they are 

 called Ccelenterata, The soft parts forming the body wall are 

 composed of two distinct membranes; there is no heart; no 

 ajDparent special respiratory organ ; and in most cases very 

 slight traces of a nervous system. 



5. All the animals not combined in the above groups, such as 

 th^ sponges, the foraminifera, and a large projDortion of the 

 Snicroscopic animalcules, form the last sub-kingdom, named 

 Protozoa. They are characterised by a general absence of any 

 special organ. 



There seems to be a much closer relationshii) between tho 

 molluscan and the j)i'otozoic sub-kingdoms than between the 

 molluscan and any of the others. It is always easier to pass 

 from the highest part of a sub -kingdom downwards in the scale 

 of nature than to pass upwards. Thus we can stejD from one 

 form to another without meeting with any marked distinction 

 from the Cephalopods to the Brachiopods, and from them to tho 



