FIXATION OF THE SEA-SQUIRT'S TADPOLE 289 



The free-swimming tadpole of the sea-squirt or Ascidian, 

 with its long, fish-like extension of the muscular body, 

 supported by its elastic backbone, with its brain and 

 spinal cord and its eye, after a short period of active life 

 in the sea runs its head against a piece of rock or other 

 solid support, and by means of an adhesive growth on 

 the skin in front of the mouth fixes itself securely (Fig. 37). 

 To this spot it remains attached for the rest of its life. 

 It is still very minute a little more than a quarter of 



TAIL 



FIG. 38. A later stage; the young Ascidian has now assumed the 

 sac-like shape of the adult. The peri-branchial chamber or cloaca 

 is formed and has its opening to the exterior. There are still only 

 two gill slits : these will multiply indefinitely as the creature grows 

 in size. The arrow shows the direction of the stream of sea-water 

 which enters the mouth and passes through the gill slits into the 

 cloacal or peri-branchial chamber and so to the exterior. The 

 actual size of the young Ascidian at this stage is about that of a 

 large hemp-seed. 



an inch long but it takes in floating particles of 

 nourishment by drawing in water to its mouth which 

 passes out by the gill-slits of its gullet, leaving the food- 

 particles behind. It grows rapidly, but not equally. The 

 tadpole's tail does not grow at all ! It simply atrophies 



19 



