CCELENTERATA — HYDROZOA 1 13 



Hydrozoons, including the Graptolithida, are abundant from 

 the Cambrian to the present. 



Derivation. — Hydrozoa > Greek hydor, water, + zoon, ani- 

 mal. 



The Hydrozoa are subdivided into the following orders : — 



Page 



T. Graptolithida 113 



Leptolinae 120 



Trachylinae 121 



Hydrocorallinae 121 



Siphonophora 121 



There is a growing tendency to make the Graptolithida a 

 sub-class of the Hydrozoa, or even to make them a distinct 

 class of the Coelenterata. They differ from the Hydrozoa 

 in the presence of (i) the sicula and (2) the virgula; also in 

 (3) the bilateral symmetry of the thecae, seen even in the sicula. 

 Since the apex of the sicula, i.e. the embryo-sheath, lacks growth 

 lines it probably possessed the radial symmetry characteristic 

 of the typical Hydrozoa. This embryo-sheath is quite similar, 

 except in composition, to the primary cup of Tabulate corals, 

 as is likewise the alternate budding of new cups (thecae). On 

 the other hand very many graptolites, Uke Diplograptus, 

 were similar to the existing hydrozoan order Siphonophora 

 in the presence of a float (pneumatocyst), and especially like 

 the living Sertularia of the hydrozoan order Leptolinae in ar- 

 rangement of thecae, presence of generative cysts (gonangia) and 

 composition of protective covering. 



Order i, Graptolithida (Graptolites) 



Type of order, Diplograptus foliaceus (fossil) (Fig. 40). 



This species is like the true hydrozoon, Sertularia, in being 

 colonial. It consists of two crowded rows of cups (hydrothecae) 

 like saw teeth, arranged in opposite series along a stalk to which 

 they are broadly but obliquely fastened. Each cup, a sub- 

 prismatic tube longer than wide, represents as in Sertularia one 

 individual polyp ; in it were lodged the soft parts and into it 

 the entire polyp could withdraw for protection. At its base 

 I 



