FOSSIL HYDROZOA. 



159 



ous poli/pites united by common flesh or ccenosarc. The cceno- 

 sarc is more or less branched (fig. 50, a), and secretes a strong 

 chitinous investment or " polypary." The polypites are also 

 protected within " hydrothecw," or little cup -like expansions 

 derived from the polypary. The pi^ocess of reproduction is 

 carried on by the development of the reproductive dements 

 irithin horny urn - like sacs, which are of larger size than 

 tlu' " hydrotheca? " and are 'known as "ovarian capsules" or 

 " gonothccm " (often called " gonophores "). Type of the order 

 the Sea-fir (Sertularia, fig. 50). 



Fig 50. o, Sertulariu (Diphasia) pinnnta, natural size ; , Fragment of the same enlarged, 

 carrying a male capsule (o), and showing the hydrothecse (A); b, Fragment of Campanularia 

 neglecki (after Hincks), showing the polypites contained in their hydrothecse (A), and also the 

 point at which the ccenosarc communicates with the stomach of the polypite (o). 



There is considerable uncertainty as to the existence of 

 any fossil representatives of this order. No undoubted 

 Sertularian, at any rate, is as yet known to the palaeontol- 

 ogist ; but there are several genera which may with more 

 or less probability be referred to this place. The most im- 

 portant of these as being those in which the reference is 

 probably correct are certain forms usually referred to the 

 (,'riijttolitida:, of which the genera Dendrograptus and Dicty- 

 onema may be noticed in particular. The forms referred to 

 Dendrograptus are exclusively confined to the Upper Cam- 

 brian and Lower Silurian formations. They consist of plant- 



