CLASS II 



HYDROMEDUSAE 



119 



The Hyrlrozoa are all aquatic, and with few exceptions are inhabitants of 

 the sea. They are commonly divided into the two following sub-classes : — 

 Hydromedusae and Acalephae. 



Subclass 1. HYDROMEDUSAE Vogt. 



Sessile or free-swimming, usually branching colonies, with dimorphic, nutritive and 

 reproductive polyps ; the latter frequently become liberated in the form of small, free- 

 sivimming Medusae, with non-lobate umbrellas composed of a hyaline, gelatinous 

 substance. 



Six orders of Hydromedusae are recognised : Hydrariae, HydrocoralJinae, 

 Tubulariae, Campanulariae, Trachymedusae and Siphonopihorae. Of these only 

 the Hydrocorallinae, Tuhulariae and Campanulariae secrete calcareous or 

 chitinous structures capable of preservation in the fossil state. 



M^m 



c~: 



Order 2. HYDROCORALLINAE Moseley.i 



Naked polyps secreting at the base a dense calcareous skeleton, traversed at intervals 

 by two series of vertical tubes, info which the dimorphic zooids can be retracted. 



The Hydrocorallinae comprise the two Recent groups Milleporidae and Stylas- 

 teridae, which were universally regarded as true corals until Louis Agassiz and 

 Moseley proved their relation- 

 ship to the Hydrozoa. 



Millepora Linn. (Fig. 185). 

 Massive, foliately expanded, 

 encrusting or branching pol}-- 

 paria (coenosteum), often attain- 

 ing considerable size. Upper 

 surface punctured by round 

 openings of the larger tubes 

 (gasfrop>ores), between which 

 are the mouths of numerous 

 smaller tubes (dacfylopores). 

 The skeleton is composed of „.„ , ,. t. . , „ ,. . 



-.J. . Millepora vodosa Bsp. Recent. A, Upper surtaee of coeno- 



a network OI anastomosing stpum, showing gastropores, A", and dactylopores c, 40/j. b, 



1 -orvi fil-ir-oo +vairovoorl Ktt- Vertical section, k, gastropores with tabulae, < ; c, Vermiform 



CalCaieOUS nores, iraverseci Oy canals communicating with dactylopores, so/j (after Stelnmann). 



a system of tortuous canals. 



The gastropores lodge the larger, nutritive polyps, and the dactylopores the 

 smaller, food-procuring zooids ; the latter have no mouths, but are provided 

 with short, clavate tentacles on their sides, and their tubes communicate with 

 the vermiform canals. Zooidal tubes tabulate, but nonseptate. The genus 

 is an important reef-builder of the present day, but occurs only sparsely in 

 the fossil state. Earliest known forms appear in the Eocene. 



1 Literature : Alhnan, J. G,, Monograph of the Gyninoblastic or Tuhuhiriau Hydroids. Ray 

 Society, 187l-72._ — Moseley, H. N., Philosophical Transactions Royal Society, vol. 167, 1878. — 

 Steininaiin, (?., Uber fossile Hydrozoeu aus der Familie der Coryniden. Palaeontographica, vol. 

 XXV., 1877.^ — Idem, Uber triasische Hydrozoen vom ostlichen Balkan. Sitzber. Akad. Wiss. 

 Wien, math. -phys. Classe, vol. cii., 1893. — Canavari, M., Idrozoi Titoniani appartenanti alia Famiglia 

 delle Ellipsactinidi. Mem. Comitato Geol. vol. iv., 1893. — Vinassa de Regny, 0. E., Studii sulle 

 Idractinie fossili. Mem, Accad. dei Lincei, 1899, ser. 6, vol. iii. 



Fig. 185. 



