METAZOA COELENTERA. 113 



Idyia roseola Ag. (No. 154) and Beroe ' ovata Esch. (No. 

 155) are elongated in form and both are without tentacles. 



Sections 3, 4. ANTHOZOA. 



ALCYONARIA. 



It is reasonable to suppose from what is already known 

 that the ancestral form of the Anthozoa possessed a simple, 

 tubular, fleshy body with a mouth at the end of an oral 

 cone, at the base of which was a limited number of solid 

 tentacles. Such a form would resemble closely the scy- 

 phostoma of the Hydrozoa, and the majority of naturalists 

 consider this as the ancestral form of the Anthozoa. If we 

 imagine the oral cone of the scyphostoma turned inward, 

 we have an internal bag hanging within the body cavity. 

 Again,- if we suppose that the fleshy walls or mesenteries 

 which are indicated in Cyanea (see p. 107) grow longer 

 and join the central bag, then we have the hydroid plan 

 of structure converted into the Actinian plan. This is 

 a crude but graphic way of illustrating the hydroid and 

 the Actinian type of structure and the possible conversion 

 of the one into the other, although it must be remembered 

 that there are no embryological facts to prove that these 

 changes actually took place. 1 



This early ancestral form probably followed essentially 

 the same path of development as the hydroid, since the 

 Actinian of to-day passes through the blastula, parenchy- 

 mella, and secondary gastrula (not invaginated) stages. 

 While, however, the parenchymellaof the hydroid is usually 

 produced by the immigration of cells from the surface to 

 the interior, that of the Anthozoa is generally made by 

 delamination of the inner ends of the ectoderm cells. 

 This process is brought about through differentiation of 



IE. B. Wilson, Phil. Trans., CLXXIV, 1883, p. 762. 



