ZOOPHYTES OF GUERNSEY. 165 



As far as concerns external form alone, the two groups, 

 the Hydroida and the Polyzoa^ are to all appearance intimately 

 related ; in fact, the similarity of their skeletons, or, to speak 

 more correctly, of the polyparies which they build up, is often 

 so close as to mislead anyone save the most practised observer. 

 But a careful examination of the living animals under the 

 microscope at once reveals structural differences which suffice 

 to separate them by a very wide interval. Both in the 

 Hydroida and in the Polyzoa we find colonies of compound 

 animals of microscopic dimensions, linked together and parti- 

 cipating in a common life ; while at the same time, each 

 member of the family enjoys its own independent and indi- 

 vidual existence. Every polyp possesses tentacles, mouth, 

 and stomach of its own, and each one has the power to seize 

 its prey, and to withdraAv within its own cell : but here its 

 indivi finality ceases ; for it is in organic communication with 

 its neighbours, and the very food it captures contributes to 

 the nourishment of the entire community. 



Xow, not only do these two groups, which Linnaeus united 

 under the Zoojjhi/ta, structurally belong to altogether different 

 divisions of the animal kingdom, but they are so widely 

 separated that whole tribes of living forms intei'vene between 

 them ; in fact, it might almost be said that they are hardly 

 related at all. The Hydroida are verv near relatives of the 

 sea-anemones which abound on oiu- shores : whereas the 

 Polyzoa, although to all outward appearance presenting many 

 points of resemblance, occu])y a much higher position in the 

 scale ; for by their structure they are closely allied to the 

 Mollusca, of which the oystei' and the cockle are well-known 

 examples. The Hydroida stand but little removed from the 

 very lowest forms of animal life, such as the Infusoria, and 

 the Sponges ; while the Polyzoa take rank above the Insects, 

 the Spiders, and the Crabs. 



Let us just glance then, very briefly, at the principal 

 characters that distinguish these two groups, which the poet 

 Montgomery speaks of, in the second canto of Pelican Island^ 

 as 



Nameless tribes, half plant, half animal, 

 Rooted and slumbering through a dream of life. 



In the Hydroid Zoophytes, or Hydroida, associated life 

 is the rule, and solitary existence the rare exception. They 

 unite to form large colonies of individuals, often numbering 

 many thousands of little animals, each of which consists of a 

 gelatinous body or sac, expanded at one end into a fleshy or 

 horny (chitinous) disc or foot, and opening at the other end 



