PHYLUM CCELENTERATA 



155 



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 hydrotheca? (Fig. 112, h.th}. 

 The coenosarcal skeleton is strengthened by a 

 slender axis, the virgida (v), 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 

 may probably be of the nature of gonothecte. 



ADDITIONAL KEMARKS 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 ; Polypodium, also an Antho- 

 medusa, found in the Volga, where in 

 one stage of its existence it is parasitic on 

 the eggs of a Sturgeon; Limnocodium, a 

 doubtful Trachymedusa, hitherto found 

 only in a tank in the Botanical Gardens, 

 Regent's Park, where it was probably in- 

 troduced from the West Indies ; and Limnocnida, found in 

 Lake Tanganyika, Africa. 



The oldest known Hydrozoa are the Graptolites, found first in 

 the Cambrian rocks ; Hydractinia occurs in the Cretaceous epoch, 

 and Hydrocorallina3 from the Cretaceous onwards. 



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

 podium, one of the Anthomedusae, 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 

 of species are independent cells, each performing alone all the 

 essential functions of an animal, others, such as Pandorina, 

 Volvox, and Proterospongia, consist of numerous unicellular 

 zooids associated to form a colony in which a certain division of 

 labour obtains, the function of reproduction, for instance, being 

 assigned to certain definite cells and not performed by all alike. 

 Thus the colonial Protozoa furnish an example of individuation, 

 numerous cells combining to form a colony in which the several 

 parts are dependent one upon another, and which may therefore 

 be said to constitute, from the physiological point of view, an 

 individual of a higher order than the cell. 



FIG. 112. Graptolites. 



A, Monograptus colonus ; 



B, Dimorphograpius, both 

 magnified ; hy. th. hydro- 

 theca ; s. sicula ; v. vir- 

 gula. (After Nicholson 

 and Lydekker.) 



