I.^„ 



HYDROMEDUSAE 



119 



The Ilydrozoa are all aquatic, and with few excepfcions 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 hranching colonies, with dimorphic, nutritive and 

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

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

 substance. 



Six Orders af Hydromedusae are recognised : Hydrariae, Hydrocorallinae, 

 Tubulariae, Campanidariae, Trachymedusae and Siphonophorae. Of these only 

 the Hydrocorallinae, Tubulariae and Campanulariae secrete calcareous or 

 chitinous structures capable of preservation in the fossil State. 



Order 2. HYDROCORALLINAE Moseley.i 



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

 by two series of vertical tubes, into which the dimorphic zoöids can be retracted. 



The Hydrocorallinae comprise the two Recent groups Milleporidae and Stylas- 

 teridae, which were univereally 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 poly- 

 paria (coenosteum), often attain- 

 ing considerable size. Upper 

 surface punctured by round 

 openings of the larger tubes 

 (gastropores), between which 

 are the mouths of numerous 

 smaller tubes (dactylopores). 

 The skeleton is composed of 

 a network of anastomosing 

 calcareous fibres, traversed by 

 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 : Allman, J. G., Moiiograph of the Gymnoblastic or Tubularian Hydroids. Ray 

 Society, 1871-72. — Moseley, II. N.. Philosopliical Transactions Royal Society, vol. 167, 1878, — 

 Steinmann, G., Über fossile Hydrozoen aus der Familie der Coryniden. Palaeontographica, vol. 

 XXV., 1877. — Idem, Über triasische Hydrozoen vom östlichen Balkan. Sitzber. Akad. Wiss. 

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

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

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



Fio. 185. 



Millepora nodosa Esp. Receiit. A, Upper surface of coeno- 

 steum, showing gastropores, k, and dactylopores c, -lo/j. B, 

 Vertioal section, k, gastropores with tabulae, t ; c, Vermiform 

 canals communicating with dactylopores, 80/j (after Steinmann). 



