138 



SYSTEMATIC ZOOLOGY 



Fin. 129. Trochophore larva of Polygordius. 

 Mouth at left, anus below, and preoral and post- 

 oral bands of cilia. (After Hatschek.) 



separate, and development 

 is always attended by the 

 formation of a larva called 

 the trochosphere, or tro- 

 chophore, or sometimes 

 Loven's larva (Fig. 129). 

 This larva is very im- 

 portant to the zoologist 

 because it resembles in 

 some characteristic points 

 the larva? of other groups 

 of animals, and so is be- 

 lieved to show a relation- 

 ship between the Annelida 

 and these other types. 



bwd 



The larva is at first more or less spherical and 

 generally provided with one or more bands of 

 cilia. The details of its structure naturally vary 

 greatly in different species of Polychceta. Asexual 

 reproduction may also occur in some genera by 

 simple transverse division (Fig. 130), while in 

 some there is a true alternation of generations. 

 In the latter case the fertilized egg develops into 

 an asexual worm, which by a sort of budding at 

 the posterior end gives rise to sexual individuals. 

 The Polychaeta are usually divided into two 

 orders, the Errantia and the Sedentaria. The 

 former move about more or less freely from place 

 to place and possess essentially all the character- 

 istics mentioned in the foregoing description. The 

 Sedentaria are more or less fixed in position and 

 generally form tubes about themselves, sometimes 

 merely particles of sand cemented together by a fig. 130. A taofytus 

 secretion from their bodies, sometimes chitinous «?/»«A«,anann^ 



lid in process of 



or calcareous structures secreted by the cells of budding; a male 

 the skin (Fig. 131). Inasmuch as the animals b "d nearly ready 



vo ^' _ to become de» 



remain permanently in these tubes, the posterior tached. (After 

 portions of their bodies become more or less "^ s f Si : ' . f L° n 



1 Parker and Ha — 



degenerate, and the parapodia and other struc- well's Text-book.) 



