234 NATURAL HISTORY OF OUR SHORES 



the sea- water, which, entering the cavity termed the Pharynx, 

 bathes the extensive and regularly fissured arrangements, 

 the " branchial sac," for respiratory purposes. 



At the same time such minute floating objects as the 

 inhaled water may contain diatoms, spores of sea-weeds, 

 larval forms of other animals, etc. are secured for food. 



Both sexes are represented in each individual, but each 

 is not self -fertilisable, fertilisation depending upon the 

 spermatozoids set free by other individuals. 



The eggs are hatched within the parent, and when ad- 

 vanced to a certain stage the young are temporarily packed 

 in a chamber, the atrium. The excretory ducts, alimentary 

 and genital, open into this chamber, whence their products 

 are expelled, together with the water, which has served its 

 purpose, through the " exhalant aperture." 



It is in these jiist liberated young that the greatest 

 interest centres, for they differ much from the parent. 



The little ones, which are of tadpole form, are active and 

 free- swimming. They possess a Notochord, that incipient 

 dorsal axis which occurs in all vertebrate embryos, and the 

 calcification of which results in the " backbone " (Fig. 102). 

 Not only have they this, but a spinal cord, which expands 

 at its anterior end into a " brain," and an eye, with retina 

 and crystalline lens. 



But after this quasi-vertebrate stage the next and final 

 one of development is what appears like reversion. 



After the little " tadpole " has led a few weeks of active 

 life it settles down, and becomes an animal of what we 

 would consider much lower status. 



At its anterior end the little tadpole has three sucker 

 arrangements, by means of which it attaches itself to 

 some support, head down, tail up. 



The tail, with its notochord (or Urochord), its pro-verte- 

 brate nerve system, brain, and spinal cord are absorbed : 

 the former disappears, the latter dwindle down to a simple 



