TUNIC AT A. 



313 



of the water gradually decreased during the night, and towards morn- 

 ing was no longer seen." 



Salpa (Fig. 125) forms another most interesting genus of 

 Tunicata ; it contains forms presenting the appearance of long 

 transparent masses of the most delicate tissue, composed of rows of 

 individuals placed side by side, and grafted, as it were, transversely — 

 ribbons, in which each animal is grafted end on end to its sister — 

 double parallel chains of social creatures, sometimes alternate, some- 

 times opposite ; living chaplets, of which each pearl is an individual. 

 Each individual presents an oblong diaphanous or prismatic body, 

 more or less symmetrical, and ofte» furnished in front, rarely behind, 

 with tentaculiform appendages. So great is their transparency, that 



r t^fi m fMi l**f»f***'r -* K^ >J*.'^.-^m^ , 





ffl 





Fig. 125. — Salpa maxima, magnified (Forsk.). 



the various organs may be observed through the skin as tliey perform 

 their several functions. 



Momus, an ancient philosopher, thought it a subject of regret that 

 Nature had not thought of piercing the body with an opening suffi- 

 ciently large for each one to see what was passing in the interior. 

 The creature which now occupies our attention would surely have 

 satisfied the demands of the philosopher — its body is, metaphorically 

 speaking, a house of glass. 



In order to move itself about the Salpa introduces water into its 

 body through a posterior opening, furnished with a valve, which it 

 expels by an anterior outlet situated near the mouth. It is thus 

 pushed backwards, and swims, as it were, by recoil. Moreover, it 

 swims with its ventral surface upwards. All the zooid forms of a 

 chain of Salpa act in concert ; they contract and dilate simultaneously ; 

 they advance as a single individual. They often float on the surface 



