g4 EMBRYOLOGY 



inasmuch as polyps which bear tentacles and become 

 sexually mature [autozooids] can be distinguished from 

 sterile individuals lacking tentacles and having only two 

 septa, the so-called zooids [siphonozooids], which provide for 

 the inflowing of the water (Wilson). 



The development of Eenilla has been investigated by 

 E. B. Wilson (No. 98). Attachment is here suppressed, and 

 by the invagination of the oesophagus and the development 

 of the septa and tentacles there is produced from the planula- 





^^^^^.^ 



dx- 



iJ 



Fig. 37.— Two stages of development of Eenilla (after E. B. Wilson). A, young 

 polyp with two polyp buds (p^) and the terminal zooid {z) ; B, central portion of a 

 somewhat older stage ; pi, p^^ p^, p*, polyp buds; z, terminal zooid; mz, marginal 

 zooid ; d2, dorsal zooid. 



larva a free-moving polypoid form (Fig. 37 A)^ which, in 

 view of the development of the colony, can be called the axial 

 individual. The upper portion of this individual persists as 

 the terminal polyp, whereas the stem of the entire colony 

 (rachis) and its lower free part, the stalk {peduncle), arise 

 from its middle and lower portions. We may also retain for 

 Renilla these terms, which are borrowed from the Pennatu- 

 lida^, because a striking similarity between these tw^o forms 

 is established in their embryology. The eight septa of the 



