VERMES. 



179 



trochosphere,^ which may be compared (in its 

 degree of development^ not in its appearance or details 

 of structure) with a medusa ; and in the Bchinodermsj 

 Brachiopods, and Molluscs, larvae, more or less resem- 

 bling this, are also found. Salanoglossus, already 

 named as a link between vertebrates and invertebrates, 

 is a worm-like animal which has a larva of this kind, 

 the special characters of which in some degree 

 approach those of the larva of Bchinoderms. 



. I'ig' 46-— o. Trochosphere Xjarva of Polygoriius, an annelid worm, shovr- 

 iDg ciliated bands, and internally the stomach with mouth to the left and 

 excretory aperture below. 



6, A later stageof development, showing the bilaterally symmetrical body 

 of the worm, consisting of successive metameres, added on as an elongation of 

 the trochosphere. 



The group of Worms is so various, and so full of 



' This kind of larva moves by means of its bands of cilia, 

 which by their action whirl it about ; hence the name trocho- 

 sphere (wheel-ball). For a detailed description of one of the 

 most important of the larval forms of worms {Polygordius, 

 an Annelid), see Parker, " Lessons in Elementary Biology," 

 Lesson xxvi. 



