IVORMS. 



A general contrast of the modes of Development in different 

 Annelids. 



A. B. 



" Larval " Types " Foetal " Types 



as in as in 



marine Chxtopods, Earthworm, Leech, etc. 

 Polygordius, etc. 



Development indirect. Development direct, within egg- 



A free-swimming Trochosphere capsule ; Trochosphere stage almost 



stage, with trunk almost or wholly or wholly suppressed. 



suppressed, with head:region greatly 



developed, with adaptations to free ^ ^^ ' -^ 



marine life. Lumbricus type Clepsiiu type 



with little nutri- with much nutri- 

 tive material in tive material in 

 ovum, with gas- ovum, with gas- 

 trula formed by trula therefore 

 invagination (em- formed by over- 

 bolic). growth(epibolic). 



By far the most important larval form among Annelids 

 is that known as the Trochosphere or Trochophore. It 

 occurs as such in the great majority of marine Chaetopods, 

 and in more or less modified guise in many other worm- 

 types, and also in Molluscs. Moreover, there are many 

 reasons for regarding the Trochosphere as to some extent 

 like the ancestor of Annelids, especially as there are a few 

 forms like Dinophilus, which do not advance far beyond the 

 structure of a Trochosphere. 



Picture a normal gastrula such as occurs in the develop- 

 ment of many marine Chaetopods. It is a two-layered sac of 

 cells with a cavity between the external ectoderm and the 

 invaginated endoderm. The blastopore or mouth of the 

 gastrula lies at the ventral pole. A ring of cilia frequently 

 forms an equatorial girdle. Now, suppose that the apical 

 (aboral) region of this symmetrical gastrula be tilted forward. 

 The ring of cilia becomes distinctly pre-oral, and internal to 

 it there may be a nervous ring. In the apex there is a 

 ganglion with a nerve running backwards. Picture a back- 

 ward growth of the posterior region which lies opposite to ' 



