INTRODUCTION. Ixi 



phora (Eschscholtz) and the Hydroida proper. The rela- 

 tionship is masked by the striking difference in habit and 

 general aspect between the two groups ; but the restless 

 ocean vagrants and the stationary, plant-like beings that 

 seem to offer the most complete contrast to them are 

 essentially identical in structure, and the leading pecu- 

 liarities of each group are only modifications of that 

 structure adapting it to various modes of life. 



At certain points of the Hydroid series, the apparent 

 dissimilarity is much less marked ; and a colony of Hydr ac- 

 tinia or Podocoryne very plainly betrays its affinity to 

 Velella or Physalia. 



Carus lias ranged the Siphonophora and the Hydroida 

 proper, as separate groups, under his order Hydromedusa. 

 Agassiz unites the two as a single order, constituting sub- 

 orders for the leading divisions of each. 



Huxley, who is followed by Greene, divides the Sipho- 

 nophora into two groups, Calycophoridce and PhysopJioridce, 

 which he regards as orders of his Class Hydrozoa, parallel 

 with the Hydridce, Corynidcs, and Sertulariadae , and with 

 the Lucernariadce, including the covered-eyed Medusae of 

 Forbes*, and Lucernaria. 



All these arrangements recognize the close affinity of 

 the Siphonophora and Hydroida, and differ only in the 

 details of their grouping. 



In the present work the first three of Prof. Huxley's 

 orders, Hydrides, Corynida, and Sertulariada, are treated 

 as suborders, and constitute together the order Hydroida. 



It seems to me clear that these divisions have no claim 

 to be considered groups of equal value with the Discophora 

 (Lucernariadce of Huxley) . 



* Steganophthalmata, Forbes; Fhanerocarpee, Each. ; Acraspeda, Gcgon- 

 baur. 



