74 MANUAL OF ZOOLOGY SECT. 



termed a colony, and the component units are termed zooids. 

 In some cases such a colony is produced by a process which 

 is more correctly termed incomplete fission than budding. 



The various systems of organs, digestive, circulatory, 

 nervous, excretory, etc., present under one form or an- 

 other in all the higher groups of animals, are variously 

 arranged and occupy various relative positions in different 

 cases, producing a number of widely different plans of 

 animal structure. According as their structure conforms 

 to one or another of these great plans, animals are referred 

 to one or another of the corresponding great divisions or 

 phyla of the animal kingdom. That animals do present 

 widely differing plans of structure is a matter of common 

 knowledge. We have only to compare the true fish, such 

 as cod, haddock, etc., in a fishdealer's shop with the 

 lobsters and the oysters, to recognise the general nature 

 of such a distinction. The first named are characterised by 

 the possession of a backbone and skull, with a brain and 

 spinal cord, and of two pairs of limbs (the paired fins) : 

 they belong to the great vertebrate or backboned group 

 the division Vertebrata of the phylum Chordata. The 

 lobsters, on the other hand, in which these special verte- 

 brate structures are absent, possess a jointed body enclosed 

 in a hard jointed case, and a number of pairs of limbs also 

 enclosed in hard jointed cases, and adapted to different 

 purposes in different parts of the body some being feelers, 

 others jaws, others legs : their general type of structure is 

 that which characterises the phylum Arthropoda. The 

 oysters, again, with their hard calcareous shell secreted by a 

 pair of special folds of the skin constituting what is termed 

 the mantle, and with a special arrangement of the nervous 

 system and other organs which need not be described here, 

 are referable to the phylum Mollusca. Other familiar 



