378 PHYSIOLOGY OF THE INVERTEBRATA. 



The PoiiiFERA OR Spongida. 



Movements in these animals are caused by the contractile 

 material of the body-substance and of the flagella ; the latter 

 being used to aid respiration, &c. The embryos of certain 

 Porifcra are richly ciliated, and thereby become free-swimming 

 larva3. 



The CtELENTP:RATA. 



The moveuTents of Hydra are performed partly by true 

 muscular fibres, and partly also by the contractions of the 

 body-wall. The tentacula are used for locomotion as well as 

 for prehension. In the Actinia', locomotion is brought about 

 by the contractions of the disc of the foot. 



Dr. G. J. Romanes* has made a thorough examination of 

 the locomotor system of the Medusa), from the standpoint of 

 experimental physiology. As these researches would fill a 

 volume in themselves, we must refer the reader desirous of 

 information on the subject to the original memoirs mentioned 

 in the foot-note. However, "it is known to every one that 

 the Mcdusw are naturally locomotive animals, the various 

 species swimming more or less rapidly by means of an 

 alternate contraction and dilatation of the entire swimming- 

 organ. It may not be so generally known that these 

 swimming-movements, although ordinarily rhythmical, are, at 

 any rate in the case of some species, to a limited extent 

 voluntary — using the latter term in the same sense as it is 

 applicable to invertebrated animals in general. For instance, 

 if Sarsia or Aurelia, &c., be gently irritated, the swimming- 

 motions immediately become accelerated, and the acceleration 

 persists for some time after the irritation has been withdrawn; 

 but to secure this result, the irritation m.ust not be of such a 

 character as an inanimate object might supply. Again, 

 individuals belonging to some discophorous species of the 



* See PhilonopJiiral Traiimctiom of the Boyal Society, 1875, P- 269; 1877, 

 p. 659; 1879, p. 161. 



