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KNOWLEDGE. 



[June 1, 1896. 



ture to hydra, or fresh-water polypes ; but ilifTor in their 

 mode of life, inasmuch as tliey live in colonies, constrnctiug 

 and inhabitiiif^' the horny polypidoms already mentioned, 

 'i'he active part of the body of the animals is con- 

 tained in the polype cells, and is provided with a 

 mouth surrounded by line contractile tentacles. 

 These are extended in the sea-water, and wave 

 about in search of tiny particles of food, which arc 

 conveyed to the mouth, and passiup; direct into 

 the body are there digested and assimilated. 



The lower part of the body of each polype 

 is prolonged through the base of the cup into 

 the tube or ccinosarc running through the stem 

 and branches of the polypidom, and is thus 

 united with the common body, to which each 

 polype contributes a share of nourishment 

 through the assimilation of food in its own 

 body. Under such conditions it would be quite 

 possible for many members of the sertularian 

 colony to live for a certain length of time 

 through the exertions of their brother polypes without 

 troubling to collect food for themselves. 



and that the weight of these naturally dried fragments 

 is -0001 of a gram, or about one five-hundredth part 

 of a grain. The tree-like polypidom from which 



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3 4'|oo'';'lNCH 



Fio. 3. — Serlularia (ihietina. 



Fig. 2. — Serfularla art/entea. 



The system of growth and reproduction in the sertu- | 

 larian colony is as follows : new polype 

 buds arise from the cccnosarc of the stem and 

 branches of the polypidom and enlarge its 

 horny substance into beads or globules. As 

 the buds develop, their horny cases open and 

 form cups, each one of which contains a fully 

 developed polype. 



Sertularia, though differing in certain 

 points, are generally considered to be the 

 modern representatives of, and closely allied 

 to, the graptolites, so common and found in 

 such variety in the lower and upper Silurian 

 formations. 



Figs. 1, 2, 3, and A are reproductions of 

 microscopic drawings of characteristic sertu- 

 larian polypidoms, showing the different form 

 and arrangement of the cells which formerly 

 contained the living animals ; and, in order that 

 the dimensions of these structures may be 

 easily determined, we have added a scale of one 

 one-hundredth of an inch to each drawing. 

 But this is not enough to convey any idea 

 of the delicate structure of the polypidoms 

 or of the vast number of polypes that formed the colony. 

 With the hope of making this more clear, we have taken 

 some of the branches of S'-rtulaiia anjentiui (Fig. 2), and 

 find that they contain one hundi'ed and seventy-eight cells, 



these were taken is nineteen centimetres (about seven 

 and a half inches) in length, and its weight is -123 

 of a gram. 



We are therefore confi'onted with the fact that 

 the whole colony — and this specimen is by no 

 means a large one, weighing when dry less 

 than two grains — must have consisted of nearly 

 fifty-five thousand individual polypes. 



In preserving these objects for future refer- 

 ence it is enough to mount them dry in shallow 

 cells formed by thin vulcanite or zinc rings. 

 They should first be thoroughly washed in warm 

 water and then allowed to become perfectly dry. 

 The cell and cover-glass should be heated over 

 a spirit lamp, so as to destroy mould spores 

 without injuring the specimen, before it is 

 hermetically sealed. Balsam and balsam and 

 benzole mounting often tend to over-clarify these 

 objects when examined under transmitted light ; 

 and air bubbles which form in the ca?nosarc 

 and polype cells — even when great care is 

 exercised to avoid their occurrence — give a false impres- 





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Fig. 4. — SerfNlan'u ^i»aAter. 



sion of the structure of the polypidom. It is true that 

 these difficulties may be overcome by the use of an air 

 pump ; and if it is desired to examine the objects under 

 polarized light, it is necessary that a much more elaborate 



