300 COELENTERATA HVDROZOA cHAr. 



a kidney-shaped swelling, the " cnidosac," or battery, which is some- 

 times protected by a hood. Each battery contains an enormous 

 number of nematocysts. In Steph,ano2)liycs, for example, there are 

 about 1700 nematocysts of four different kinds in each battery. At 

 the extremity of the battery there is a delicate terminal filament. 

 The action of the battery in Stephanophyes is, according to Chun,^ 

 a very complicated one. The terminal filament lassos the prey 

 and discharges its somewhat feeble nematocysts at it (Fig. 141). 

 If this kills it, the dactylozooid contracts and passes the prey 

 to a gastrozooid. If the animal continues its struggles, it is 

 drawn up to the distal end of the battery and receives the 

 discharge of a large number of nematocysts ; and if this also 

 fails to put an end to its life, a membrane covering the largest 

 and most powerful nematocysts at the proximal end of the 

 whole battery is ruptured, and a final broadside of stinging 

 threads is shot at it. 



The larger nematocysts of these batteries in the Siphonophora 

 are among the largest found in Coelenterata, being from 0"5 to 

 0"1 mm. in length, and they are frequently capable of inflicting 

 painful stings on the human skin. The species of Physalia, 

 commonly called " Portuguese Men-of-War," have perhaps the 

 worst reputation in this respect, the pain being not only intense 

 but lasting a long time. 



Hydrophyllia. — In many Siphonophora a number of short, 

 mouthless, non-sexual zooids occur, which appear to have no 

 other function than that of shielding or protecting other and 

 more vital parts of the colony. They consist of an axis of firm 

 mesogloea, covered by a layer of flattened ectoderm, and they 

 may be finger-shaped or triangular in form. In Acjalma and 

 Praya an endoderm canal perforates the mesogloea and terminates 

 in a little mouth at the free extremity. In Athoria and 

 Bhodop)hysa the hydrophyllium terminates in a little nectocalyx. 



Pneumatophore. — In all the Siphonophora, with the exception 

 of the Calycophorae, there is found on one side or at one 

 extremity of the colony a vesicle or bladder containing a gas,^ 

 which serves as a float to support the colony in the water. 



1 Ahh. Scnckcnh. Gcs. xvi. 1891, p. 44. 



- This gas is frequently called air. The gas contained in the pneumatophore of 

 Physalia was analysed by Schloessing and Richard. C.R. c\xii. 1896, p. 615, and 

 found to consist of C0„ 1'7 parts, l.')'!, nitrogen and argon, 83-2. 



