CNIDARIA. 



123 



A few are parasitic, e.g., Polypodium hydriforme parasitic in the young stage 

 in the ova of the Sterlet (Acipenser ruthenus), (Ussow, Ann. and Mag. N". Hist. 

 (5), 18, 1886); and Mnestra, a medusa parasitic on Phyllirhoe. 



The few fossil forms known belong for the most part to the corals, 

 the hard tissues of which have left traces in the rocks of most periods. 

 There are traces of Medusa remains in the Solenhofen slates, and the 

 Graptolites of the Cambrian formation may possibly have been 

 hydroid colonies, resembling the Sertularidae. 



The following table shows the classification of the Coelenterata 

 adopted in this work : 



Sub-phylum I. CNIDARIA. 



CLASS I. HYDROMEDUSAE 



(CRASPEDOTA). 

 Order 1. HYDRIDA. 

 2. HYDROOOKALLINAE. 

 ,, 3. TUBULARIAE. 



ANTHOMEDUSAE. 

 ,, 4. CAMPANTJLARIAE. 



LEPTOMEDUSAE. 

 ,, 5. TRACHOMEDUSAE. 

 ,, 6. NARCOMEDUSAE. 

 7. SIPHONOPHORA. 



CLASS II. ACALEPHAE 



(ACRASPEDOTA). 

 Order 1. SCYPHOMEDUSAE 



(TETRAMERALIA). 



CLASS II. ACALEPHAE Continued. 



Order 2. EPHYRONIAE 



(OCTOMERALIA). 



CLASS III. ANTHOZOA. 



Order 1. RUGOSA 



(TETRACORALLA). 

 ,, 2. ALCYONARIA 



(OCTACTINIA). 

 ,, 3. ZOANTHARIA 



(HEXACTINIA). 



Sub-phylum II. CTENOPHOBA. 



CLASS CTENOPHORA. 



Order 1. TENTACULATA. 

 2. NUDA. 



Sub-phylum I. CNIDARIA. 



Coelenterata with thread-cells. 



It is on the whole convenient to use the characteristic thread-cells 

 to divide the Coelenterata into two sub-phyla the Cnidaria and 

 Ctenopliora although it must not be forgotten that these structures 

 have been observed in one Ctenophore, and are found outside the 

 limits of the Coelenterata altogether (Platyhelminthes, Mollusca, 

 Protozoa). 



Class I. HYDROMEDUSAE (CRASPEDOTA).* 



Cnidaria in which the medusa has a velum and the polyp is 

 without gastral ridges or filaments. 



This class includes polyps; colonies of polyps, which produce 



* The Hydroniedusae coincides with the old group Hydrozoa, excluding the 

 Acalephae. It appears convenient to place the latter in a separate class to mark 

 their intermediate position between the Hydromedusae and Actinozoa. 



