42 



OUR COMMON BRITISH FOSSILS. 



just separating the individual zoophytes. The horny 

 matter, it must be remembered, is secreted by the soft, 

 simple flesh (coenosarc). At certain times there will 

 be borne on the branches, horny capsules, much larger 

 than usual. These are the gonothecce, for the special 

 purpose of reproduction. The young ova issue hence 

 as little free-swimming animals ; some of them to 

 assume, during their wandering life — and before they 

 settle down to bud and produce a " sea-fir " colony 



Fig. 30.— Magnified calycles (a) and capsule {h) of common sea-fir {Sertularia 

 alnetina), 



— the appearance, and partly also the structure, of 

 jelly-fish. 



Now in many respects the Graptolites resembled 

 recent Sertularians. First, they were composed of 

 a similar horny or chitinous external substance, which, 

 indeed, is all that is left of them in the fossil state, 

 just as the entangled masses of the " sea-firs " so often 

 picked up along the coast and mistaken for seaweeds, 



