MANUAL OF NATURAL HISTORY. 339 



irregularly oblong, flat above, with a ver- 

 tical lamina on the upper surfaca 



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 (Polytomidae). Body com- 



posed of numerous pieces aggregated together. 



V. ORDER. — Ciliograde Sea-Nettles (Ciliogradag). 



Body gelatinous, free, marked on the surface with 

 narrow ambulacra formed by rows of vibratile cilia. 



1. Family. — Beroes (Beroidae). 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-Beroes (Callianiridse). Body 



with projecting ciliated ribs, united together 

 and forming lateral wings. In Cesium the ribs 

 are two, and the wings excessively prolonged. 



q2 



