110 HYDROZOA. 



The polypites of the several genera differ chiefly 

 in size and mode of attachment. They may be 

 inserted in continuous series, along one side of 

 the coenosarc only, as in Stephmiomia, or indif- 

 ferently on either side, as exemplified by Rhizo- 

 jjhysa. In some cases they are attached directly 

 to the coenosarc ; in others, supported, with their 

 tentacles and hydrophyllia, upon special stalks. 

 In Apolemia they are arranged in groups of two 

 or three, along with the other appendages, at 

 intervals, as above mentioned ; in Physophora 

 and Athoryhia they form a spiral or circlet 

 around its distal extremity, while in Physalia, 

 Velella and Porpita, they are restricted to the 

 inferior surface of the much modified hydrosoma. 



Two kinds of polypites, a larger and a smaller, 

 appear on the same coenosarc in certain genera. 

 Much doubt exists as to the true nature of the 

 latter, which in some cases appear to have been 

 mistaken for hydrocysts, in others, for gonoblas- 

 tidia. In Agahna the smaller polypites often 

 equal in length the true digestive zooids, but are 

 always much narrower, the rudimentary tentacle 

 at the base of each presenting a striking contrast 

 to the highly complex prehensile organs attached 

 to the pedicles of the polypites properly so called. 



The hydrocysts, though present in many other 

 genera, are especially conspicuous in Physophora 

 and Athoryhia. In the former they are disposed, 

 in a circular series, external to the polypites, 

 around the expanded distal extremit}?- of the 

 coenosarc, and, from their bright pink colour 

 and larger size, are more noticeable, on a first 

 inspection, than the true digestive appendages. 

 {fig. 22, h.) 



