166 



ZOOLOGY 



SECT. 



is still quite legitimate to describe them in the same terms as the 

 other Siphonophora i.e. to consider them as hydroid colonies in 

 which the cosnosarc is represented by the discoid or rhomboid 

 body with its contained air-chamber. 



ORDER 5. GRAPTOLITHIDA. 



The '* Graptolites " are fossil Hydrozoa found in the Upper Cambrian and 

 Silurian rocks. They are known only by their fossilised chitinoid skeleton, all 

 trace of the soft parts having, as in the majority of fossils, disappeared. 



With one doubtful exception they are compound, consisting of an elongated 

 tube the perisarc of the common stem, having attached to it, either in a single 

 or a double row, numerous small projections, the hydrothecse (Fig. 126, h.lh). 

 The coenosarcal skeleton is strengthened by a slender 

 axis, the virgula (t>), the proximal end of which is 

 connected with a small dagger-shaped body, the 

 sicula (s), supposed to be the skeleton of the 

 primary zooid by the budding of which the colony 

 was produced. In connection with some species 

 oval or cup-like capsules have been found : these 

 maj be of the nature of gonothecse. But it must 

 be added that the evidence in favour of associating 

 the Graptolites with the Hydrozoa is by no means 

 conclusive, and reasons have been adduced for 

 regarding them as connected with groups much 

 higher in the scale. 



ADDITIONAL REMARKS ON THE HYDROZOA. 



The vast majority of Hydrozoa are marine, 

 the only exceptions being Hydra, found all 

 over the world ; Microhydra, at present 

 known only in North America ; Cordylophora, 

 one of the Anthomedusae, found in Europe, 

 America, Australia, and New Zealand ; Poly- 

 E,Dimorphdgraptus,both podium, also an Anthomedusa, found in the 



magnified, hy. th. hydro- \ T -, -, . , ., . . ... 



theca ; s. sicuia ; v. vir- Volga, where in one stage of its existence it is 



IndLydtkker.) Nic son parasitic on the eggs of the Sturgeon ; Limno- 



codium, a doubtful Trachymedusa, hitherto 



found only in a tank in the Botanical Gardens, Regent's Park, 



where it was probably introduced from the West Indies ; and 



Limnocnida, found in Lakes Tanganyika and Victoria Nyanza and 



in the river Niger. 



The oldest known Hydrozoa are the Graptolites, found first in 

 the Cambrian rocks ; Hydractinia occurs in the Cretaceous epoch, 

 and Hydrocorallinae from the Cretaceous onwards. 



Parasitism, although rare, is not unknown in the class. Poly- 

 podium, one of the Anthomedusse, is parasitic during part of its 

 existence, in the ovary of the Sturgeon ; and Cunina, one of the 

 Narcomedusse, is parasitic on a Trachymedusa. 



In the section on the Protozoa we saw that while the majority 



FIG. 126. Graptolites. 



A, Monograplus colonus 



