COELENTERATA AND CTENOPHORA 



Fig. 10.10. A siphonophoran, 

 Physalia. The entire colony is 

 suspended below the float. Sev- 

 eral small fishes have been 

 snared and killed by dactylo- 

 zooids and are being drawn 

 up for ingestion and digestion 

 in gastrozooids. (Photograph 

 by George Lower.) 



colonial, and shows a remarkably telescoped pattern of development, without 

 medusae or planulae. It is preferable to assume that planula-like ancestors 

 developed into medusae, and that the polypoid stages represent original larval 

 forms which have persisted and become, in many cases, reproductive stages. 

 Other Hydrozoa. Related to the hydroids and hydromedusae are the 

 hydroid corals, or Hydrocorallinae. These have a massive skeleton of car- 

 bonate of lime, somewhat resembling the skeleton of true corals. Other 

 interesting types are the Siphonophora, shown by the fossil record to be an 

 extremely ancient group, of which the "Portuguese man-of-war," Physalia, is 

 the most familiar example (Fig. 10.10). Physalia is a colony of specialized 

 individuals, sometimes spoken of as "persons," having a gas-filled float sup- 

 porting the whole. The colony closely resembles an individual organism, with 

 organs specialized for various functions such as food-getting, digestion, repro- 

 duction, and so on. Actually, the "organs" are individual members of the 

 colony, each specialized to perform a particular function. Such a group may 

 be spoken of as polymorphic, in contrast to dimorphic colonies like those of 

 Obelia. The individuals that function as tentacles (dactylozooids) are laden 

 with nematocysts which can affect the human skin very severely. These 

 "persons" capture such prey as small fishes and crustaceans, which are then 

 drawn up to the digestive polyps, or gastrozooids, near the float. There are 



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