SUB-CLASS I HY DROZOA—HY DROMEDUSAE 109 
Sub-Class 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- 
swimming Medusae, with non-lobate umbrellas composed of a hyaline, gelatinous 
substance. 
Six orders of Hydromedusue are recognised — Hydrarie, Hydrocorallinae, 
Tubulariae, Campanulariae, Trachymedusae, and Siphonophorae. But of these only 
the Hydrocorallinae, Tubulariae, and Campanulariae secrete durable, calcareous, or 
chitinous structures. 
Order 2.5 HYDROCORALLINAH. Moseley. * 
Naked polyps secreting at the buse a dense calcareous skeleton, traversed ut intervals 
by two series of vertical tubes, into which the dimorphic zooids can be retracted. 
The Hydrocorallinue comprise the two recent groups Milleporidue and Stylas- 
teridue, which were universally regarded as true corals until Louis Agassiz and 
Moseley proved their relationship to the Hydrozou. 
Millepora, Lin. (Fig. 193). Massive, foliately expanded, encrusting, or 
branching polyparia (coenosteum), often attaining considerable size. Upper 
surface punctured by round open- 
ings of the larger tubes (gastro- B 
pores), 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 zodids ; the latter have 
no mouth, but are provided with 
short, clavate tentacles ean their Millepora nodosa, Esp. Recent. A, Upper surface of coeno- 
sides, and their tubes communi- steuin, showing gastropores, k, and dactylopores, ¢, 49/;._ B, Ver- 
. Oran tical section, h, gastropores with tabulae, ¢; c, Vermiform canals 
cate with the vermiform canals. communicating with dactylopores, 59/; (after Steinmann). 
Zobidal tubes tabulate, but non- 
septate. 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 Kocene. 
Stylaster, Gray. Branching polyparia composed of a network of fibrous, 
rose-coloured coenenchyma, in which are situated calicular depressions that are 

Fig. 193. 
1 Allman, J. G., Monograph of the Gymnoblastic or Tubularian Hydroids ; Ray Society, 1871-72. 
—Steinmann, G., Ueber fossile Hydrozoen aus der Familie der Coryniden (Palaeontographica, Bd. 
XXV.), 1877.—Ueber triasische Hydrozoen vom Ostlichen Balkan (Sitzungsber. Wiener Akad. 
math. phys. Classe, Bd. CIL.), 1893.—Canavari, M., Idrozoi Titoniani appartenanti alla Famiglia 
delle Ellipsactinidi (Mem. Comitato Geol. vol. IV.), 1893.—WNicholson, H. A., Monograph of the 
British Stromatoporoids (Palaeontographical Society), 1886-92.—Bargatzhi, A., Die Stromatoporen 
des rheinischen Devons. Bonn, 1881. 
2 Moseley, H. N., Philosophical Transactions Royal Society, vol. 167, 1878. 
