PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



121 



Figure 59. Portuguese man-of-war. The Portu- 

 guese man-of-war floats on the surface of tropical 

 seas. Hanging down from the float are long tentacles 

 loaded with nematocysts. Many fish are captured 

 by these streamerlike tentacles. The colony shown 

 has just caught a fish; arrow points to fish. How 

 ever, one species of fish of the genus Nomeus swims 

 about among the tentacles of the Portuguese man- 

 of-war with impunity. It appears to be immune to 

 the poison of the stinging cells, possibly because it 

 eats the tentacles of its host. These fish dart out to 

 grasp a small food animal and hasten back amid 

 the safety of the tentacles to devour it. The tenta- 

 cles protect the fish, and particles of food not eaten 

 by the fish are engulfed by the Portuguese man-of- 

 war. (Courtesy of N.Y. Zoological Society.) 



locomotor organs but is carried from place 

 to place by currents in the water or by 

 winds blowing against the pneumatophore. 

 The stinging dactylozooids may be over 60 

 feet in length, with nematocysts powerful 

 enough to inflict serious and even fatal in- 

 jury on man. The dactylozooids are able to 

 catch large fish; and by means of contrac- 

 tion, they draw them up to the gastrozooids; 

 these enclose the prey in a digestive sac by 

 spreading their lips over it. 



Aurellia— a scyphozoan 

 medusa 



Most of the larger jcllyfishcs belong to the 

 class Scyphozoa. They can be distinguished 

 easilv from the hvdrozoan medusae by the 

 presence of notches, usually 8, in the margin 

 of the umbrella, and by the absence of a 

 distinct velum. The scyphozoan jellyfishcs 

 usually range from an inch to 3 or 4 feet in 

 diameter. The giant jellyfish, Cyanea, which 

 lives in the cold water of the north Atlantic, 

 has been found with a disk up to IVi feet 

 in diameter, tentacles 120 feet long, and 

 weighing up to one ton. The Scyphozoa are 

 usually found floating near the surface of the 

 sea, though some of them are attached to 

 rocks and seaweeds. There is an alternation 

 of generations in their life history, but the 

 asexual stage is subordinate. 



Aurellia (Fig. 60) is white or bluish, with 

 pink gonads. It differs from Gonionemus 

 and other hvdrozoan medusae in the ab- 

 sence of a velum, the characteristics of the 

 canal system, the position of the gonads, 

 and the arrangement and morphology- of the 

 sense organs. 



The oral arms hang down from the square 

 mouth, which opens into a short gullet; 

 this leads to a rectangular central enteron. 

 Gastric pouches extend laterally from 4 

 sides of the enteron. Within each gastric 

 pouch is a gonad and a row of small gastric 

 filaments bearing nematocysts. Numerous 

 radial canals, some of which branch several 

 times, lead from the enteron to a ring canal 

 at the margin. The 8 sense organs of Aurel- 

 lia lie between the marginal lappets and 

 are known as tentaculocysts; these are con- 

 sidered to be organs of equilibrium. In 

 addition, each tentaculocyst bears a pigment 

 spot which is sensitive to light. 



The food of Aurellia consists of small 

 particles which are carried along the radial 

 canals by currents produced by the beating 

 of cilia with which some of the gastrodermal 

 cells are provided and are ingested by gastro- 

 dermal cells. The physiologic processes in 



