2 MANFAL OF THE MOLLTJSCA. 



such as Conularia, are ricli in the phosphate of lime. The 

 digestive cavity is completely separated from, the walls of the 

 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 

 Heterogangliata for the great group of Mollusca. The end of 

 the alimentary canal nearest the mouth is surrounded 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 suflBciently 

 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. 

 The propriety of arranging worms with insects will be seen, if 

 it be remembered that even the butterfly and bee commence 

 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. From this equal lateral development of the nervous 

 centres Professor Owen calls the group Homogangliata. The 

 nervous system is traversed by the alimentary canal. The 

 radiated animals form a part of this sub-kingdom. 



4. The next sub-kingdom comprises 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 apparatus, on which account they are 

 called Coelenterata. The soft parts forming the body wall are 

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

 apparent 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 

 the sponges, the foraminifera, and a large proportion of the 

 microscopic animalcules, form the last sub-kingdom, named 

 Protozoa. They are characterised by a general abse!iice of any 

 special organ. 



There seems to be a much closer relationship between the 

 molluscan and the protozoic sub -kingdoms than between the 

 moUuscan 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 step from one 

 form to another without meeting with any marked distinction 

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



