ANIMAL LIFE PAST AND PRESENT. 



forwards, the result of which is to propel the anirrjal 

 backwards. Minor aid in swimming is afforded either 

 by expansions of the skin on the sides of the body, or 



by distinct fins near the tail ; 

 while many of these creatures 

 aid their escape from foes by 

 the sudden discharge of an 

 inky fluid during their back- 

 ward course. 



Our notice of swimming 

 Invertebrates cannot be con- 

 cluded without mention of 

 those curious marine animals 

 known as Sea -Squirts, and 

 technically as Tunicates a 

 group usually placed in the 

 neighbourhood of the Mol- 

 luscs. Although in the adult state many of the 

 Tunicates exist in the form of the bag-like squirts 

 with which many of us are familiar, yet all are free- 

 swimming creatures in the young condition. Moreover, 

 certain of them, like the Salpae, are pelagic through- 

 out their existence ; some of the latter forming chains 

 composed of numerous individuals attached to one 

 another. These Salpse-chains vary in length, from a 

 few inches to several feet, and swim on the ocean 

 surface with a serpentine movement. The great 

 interest attaching to these Tunicates is that the young 

 exhibit certain structures closely simulating the primi- 

 tive condition of the spinal column of Vertebrates, and 



FIG. If). Shell of Ammonite 

 (After Gaudry.) 



