TYPE CCELENTERA. 115 



depths of the ocean the simple forms which do not produce colonies are 

 frequently found. 



Relationships of the Anthozoa. As has been pointed out, it seems prob- 

 able that the Anthozoa are to be traced back to a Scyphostoma-like polyp 

 lacking interradial funnels. Nofour-mesenteriedfonn, however, is known, 

 a large gap existing between the Scyphostoma and the Alcyonaria, which 

 are probably the simplest Anthozoa known to us. The primitive Alcyonaria 

 were undoubtedly simple forms, and from them to the Edwardsise was not 

 a very great step. By the formation of four additional mesenteries the 

 Ivhvardsian condition became converted into the twelve-mesenteried con- 

 dition which forms the ground-form of the Protactinia?, Zoanthea?, and 

 Hexactinia3, the various stages seen in the Protactinise indicating the 

 manner in which, the Hexactinian condition has been brought about. 

 The Cerianthese seem to be offsets from the Edwardsian condition, but it is 

 difficult in the present state of our knowledge to conjecture the affinities of 

 the Antipatharia. 



It is noticeable that the members of all the orders except the Hexac- 

 tmiae have a strictly bilateral arrangement and development of the mesen- 

 teries ; this arrangement becomes gradually modified, first, by the ten- 

 dency of the mesenteries to arrange themselves in pairs ; second, by the 

 formation of secondary mesenteries ; third, by a tendency for these to 

 appear in pairs ; fourth, by a tendency for such pairs to appear in all the 

 intervals between the primary pairs. Thus the Anthozoa are forms which 

 are gradually specializing away from the radial symmetry characteristic 

 of all Coelenterales towards a bilateral symmetry, and the more pronounced 

 radiality of the Hexactiniae is a secondary condition. 



SUBKINGDOM METAZOA. 

 TYPE CCELENTERA. 



I. Subtype PORIFERA. With pores in the walls and without nemato- 

 cysts. 



1. Order Calcarea. Skeleton calcareous. 



(a) Ascon type. Leucosolenia. 



(b) Sycon type. Grantia. 



2. Order Cornacuspongice. Skeleton of spongiolin, usually with 



simple siliceous spicules. 



(a) With spicules ; fresh water. Sponyilla, Ephydatia. 



(b) Without spicules ; marine. Euspongia. 



3. Order Spicidispongm. Skeleton of uniaxial or tetraxial sili- 



ceous spicules. Sometimes entirely wanting. 



(a) Skeleton wanting. Halisarca. 



(b) Skeleton present. Cliona, Esperella. 



4. Order Hyalospongice. Skeleton of 6-rayed siliceous spicules. 



Euplectella. 



