168 



INVEBTEBBATE MOBPHOLOQT. 



front of the mouth and two behind it, which gradually separate from the 

 ectoderm to form four single-layered plates lying immediately beneath it. 

 By a subsequent growth and fusion of these plates a new ectodermal'cover- 

 ing is formed enclosing the internal organs, and on its completion the orig- 

 inal larval ectoderm is thrown off. In some species a somewhat more com- 

 plicated process occurs. The larva, known as the Pilidium (Fig. 85), has 

 the shape of a helmet from whose rim two ear-like lappets hang down, be- 



FlG. 85. — Filidium Larva (after Salenskt). 

 ap = apical plate. m = mouth. s = digestive sac. 



tween which lies the mouth-opening (m), while at the apex of the helmet 

 there is an ectodermal thickening (ap), nervous in character, from which 

 projects a bunch of strong sensory cilia. As in the Desor larva four invagi- 

 nations of the ectoderm of the ventral surface occur, which, however, sepa- 

 rate from the larval ectoderm as four hollow sacs which unite together, 

 their inner walls thickening to form the ectoderm of the young Nemertean, 

 while the outer walls become thin and form what is termed the amnion sur- 

 ■rounding a cavity within which lies the young worm. During the process 

 of fusion of the four sacs the enteron ( 8) and a portion of the mesoderm 

 of the Pilidium are enclosed and give rise to the digestive tract and meso- 

 derm of the young worm, which later breaks through the amnion and 

 Pilidium wall to become free. 



The significance of this metamorphosis is decidedly obscure. Some 

 authors regard it as more primitive than the direct method of development, 

 on the ground that the Pilidium with its lappets presents general simi- 

 larities to the MtiUerian larva of the Polyclades and is derived phylogeneti- 

 cally from such a form, being therefore more ancestral in its eharacters 

 than the simpler larvse. It must be recognized, however, that there is no 

 indication of metamorphosis in the Polyclad larvte, and furthermore that 



