ISO 



HARDWICKE'S SCIENCE-GOSSIP. 



others, in the possession of characters which cause 

 Professor Allman, the great authority on the 

 Hydrozoa, to regard them as intermediate between 

 Hydrozoa and Rhizopoda. 



In the e: Sea-firs " (Gg. 117), we have a horny stem, 

 hollow throughout, giving off branches, like a minia- 

 ture tree. These branches are also hollow, and 

 communicate with small cups called HydrotJieccc. 

 In each of the latter a distinct zoophyte lives, 

 capable of putting forth its fringe of tentacles 



Fig. 117. Coir.mon Sea-fir {Sertularia abietina). 



beyond the rim, and of withdrawing them again. 

 Each individual is connected by means of the 

 simple fleshly tissue (ccenosarc) which fills the 

 hollow stems and branches, with every other on the 

 same colony or polypidom. At the base of each of 



Fig. 118. Sertu'aria cupres*inti magnified ; a, calycles; 

 b, gonotlieca or generative capsule. 



the small cups is a partition, just separating the in- 

 dividual zoophytes. The horny matter, it must be 

 remembered, is secreted by the soft, simple flesh 

 {ccenosarc). At certain times there will be borne 

 on the branches horny capsules, much larger than 

 usual. These are the gonotheccc, 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 — the appearance, and partly, also, 

 the structure, of jelly-fish. 



Fig. 119. Magnified calycles and capsule of common Sea-fir 

 {Sertularia abietina'). 



Now, in many respects, the Graptolites resembled 

 the modern Sertulariaiis. First, they were com- 

 posed of a similar horny : or chitinous external sub- 

 stance, which, indeed, is all that is left of them in 

 the fossil state, just as the entangled masses of 

 "sea-firs" so often picked up along the coast and 

 mistaken for seaweeds, are all that is left of the 



Fig. 120. Double Graptolite (Didymograpsvs Murehisonia). 

 Fig. 121. Calycles ol ditto, magnified. 



living colony of which they formed the mere solid 

 and endurable parts. The Graptolites, also, were 

 like the Sertttht rictus in being compound animals, 

 or rather, a colony of simple, hydra-like creatures ; 

 whence the name of IJijdroida. In the Graptolites, 

 however, the homy cups are crowded closely toge- 



