IV 



PHYLUM CCELENTERATA 



167 



or a double row, numerous small projections, the hydrothecae (Fig. 125, h.th). 

 Tlic ccviiosarcal skeleton is strengthened by a slender axis, the viryida (v), the 

 proximal end of which is connected with a small dagger-shaped body, the 

 •tenia (•<), supposed to be the skeleton of the primary 

 zooid by the budding of which the colony was pro- 

 duced. In connection with some species oval or 

 cup-like capsules have been found : these may be of 

 the nature of gonothecse. But it must be added that 

 the evidence in favour of associating the Oraptolites 

 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 arc 

 marine, the only exceptions being Hydra, 

 found all over the world ; Microhydra, at 

 present known only in North America; 

 Cordylophom, 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 . Zimnocnida, found 

 Lake Tanganyika, Africa. 



The oldest known Hydrozoa are the Graptolites, found first in 

 the Cambrian rocks ; Hydractinia occurs in the Cretaceous epoch, 

 and Hydrocorallinse from the Cretaceous onwards. 



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

 podium, one of the Anthomedusoe, 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. 



125. — Graptolites. 



A, Monofiraptus colonics; 



B, DimorphoaraptUB, both 

 magnified, hy. th. hydro- 

 theca ; s. sicula ; v. vir- 

 gula. (After Nicholson 

 and Lydekker.) 



Ill 



