104 



INVERTEBRATE MORPHOLOGY. 



EpHyra seems to represent an ancestral stage, since some mature medusae 

 resemble this stage very closely and it occurs in the life-history of all, and 

 earlier than this is the Scyphostoma representing the Lucernaria stage of 

 evolution. The Scyphostoma has superficial resemblance to a hydroid 



FiQ. 55.- 



-A, Scyphostom of Aurelia ; B, Strobila of Aurelia ; C, Ephyra of 

 Pelagia (all after Agassiz). 



polyp, which resemblance is almost an identity in the earlier stages of the 

 Scyphostoma before the development of the mesenteries and funnels. This 

 suggests a relationship of the Scyphomedusas to the Hydromedusaa only 

 through the polyp, the separation of the two classes having occurred be- 

 fore the appearance of the medussB on the scene. 



III. Class Anthozoa. 



The Anthozoa never assiime the medusa form, but are ses- 

 sile, usually colony-producing polyps of the Scyphostoma 

 type. Typically they are cylindrical structures (Fig. 56) at- 

 tached at one extremity, the hase, and bearing at the other 

 extremity the mouth in the centre of a flat surface, the disk, 

 around the margins of which are a number of hollow tenta- 

 cles. The coelenteron is imperfectly divided into a number 

 of chambers by longitudinal partitions arising from the body- 

 wall, the mesenteries (Fig. 57, me), the various intermesenterial 



