MANUAL OF NATUKAL HISTORY. 339 



irregularly oblong, flat above, with, a ver- 

 tical lamina on the upper surface. 



IV/. ORDER. DIPHYDIAN SEA-NETTLES (Diphydse). 



Body bilateral, symmetrical, with two or more 

 hollow, contractile, sub-cartilaginous swimming or- 

 gans, placed one before the other ; an ovigerous fila- 

 ment prolonged posteriorly. 



1. FAMILY. Diphyds-Proper (Diphyidse). Body 



composed of two pieces adhering together, and 

 capable of separation. 



2. FAMILY. Polytomes (Polytomidse). Body com- 



posed of numerous pieces aggregated together. 



V. ORDER. CILIOGRADE SEA-NETTLES (Ciliogradse). 



Body gelatinous, free, marked on the surface with 

 narrow ambulacra formed by rows of vibratile cilia. 



1. FAMILY. Beroes (Beroidse). Body symmetrical, 



terminated at each pole by an opening, with 

 ciliated ribs and two ciliated tentacles arising 

 from the inferior extremity. 



2. FAMILY. Alcynoes (Alcynoidse). Body cylindri- 



cal, open at one end, furnished at the other 

 with two large wings which often envelope 

 the entire body ; ribs of body ending in a 

 point. 



3. FAMILY. Winged-Heroes (Callianiridse). Body 



with projecting ciliated ribs, united together 

 and forming lateral wings. In Cesium the ribs 

 are two, and the wings excessively prolonged. 



Q 2 



