CCELENTERATA — JELLY-FISH 1 2 1 



tions of themselves, which usually become free-swimming 

 jelly-fish (medusae) ; these in turn produce eggs and sperm, the 

 union of which gives rise again to a fixed generation. Known 

 from the Mesozoic to the present. Living examples are Obelia 

 and Sertularia (see p. io8). 



Order j, TrachylincB. — No fixed generation discovered ; all 

 are free-swimming medusae. No fossils known. 



Order 4, HydrocorallincB. — Colonial Hydrozoa, the common 

 base of the colony secreting a massive calcareous support. 

 Known from the pre-Cambrian to the present. Millepora is a 

 living genus, while the species of Stromatopora were very abun- 

 dant and important limestone builders in the Paleozoic. 



Order 5, Siphonophora. — Colonial, pelagic Hydrozoa, with 

 remarkable diversity of form among the various individuals 

 in a colony. They are often supported by a float of their own 

 manufacture. Not known in the fossil state. Example, 

 Portuguese man-of-war (Physalia). 



CLASS B, SCYPHOZOA (JELLY-FISH) 



Large, free-swimming, umbrella-shaped jelly-fish (medusae). 

 The margin of the umbrella is usually lobed, bearing tentacles, 

 w^hile the mouth occupies the position of the umbrella's handle. 

 They develop directly from the egg or by the alternation of a 

 sessile stage; in the latter case they multiply by transverse 

 fission. They swim by rhythmically opening and closing their um- 

 brella-like bodies. Their food consists of small fish, crustaceans, 

 and even of each other. None live longer than a year. 



The Scyphozoa are known to have existed from the Cambrian 

 (Fig. 45) to the present. They are entirely without hard parts 

 and consist of about 99 per cent water, so that only under ex- 

 ceptional conditions have they left a record of their presence. 

 At times in very fine-grained muds, as in the lithographic slates 

 of Bavaria (of Jurassic age) impressions of their soft bodies are 

 preserved, or even imprints of their tentacles trailing over the 

 yielding mud. At other times the lobed digestive cavity has 



