HYDROZOA 



FTIHE HYDROZOA form one of the three classes into 

 J_ which the Codentera nematophora (distinguished from 

 the Codentera porifera, or Sponges) have been divided. 

 It results from observations made by Ernst Haeckel that 

 the Ctenopkora should not be regarded as a class equi- 



valent to the Hydrozoa and Actinozoa, nor as a subdivision 

 of the latter class, but that they must be considered as a 

 peculiar modification of the medusiform Hydrozoa (see 

 final paragraph). If this conclusion be accepted, it will 

 be necessary to divide the Hydrozoa into two primary 



Scyphomedusas from the Deep Sea. (After Haeckel, Challtager Rtportt, vol. iv. 1882). 



A. feriphflla minM2tt, Haeck., one of the Peromednsse, one-third the natural size, a, one of the font interradial tentaculocysts (sensory organs) sunk 



between its lappets ; 6, one of the sixteen snbradial coronal lobes. The twelve tentacles (four perradial. eight ail radial) are seen. 



B. Perradial section through Luarnaria baUiyphila, Haeck., nat. size, a, perradial gastral poach ; 6, gasrral aiial cavity ; c, ovary (four); d, gasrral filaments; 



e, perradial gastral pouch ; /, manubrium and mouth ; 0, the bunches of tentacles (eipht, adradial). 



The eight principal tentacles (four perradial and four interradial) are not in this species converted Into adhesive anchors as In L. auricula, but are 

 altogether suppressed. 



groups or grades, for which the names Polypomorpha and 

 Ctenophora are proposed. 



The Hydrozoa correspond to the Linnsean genera Hydra, 

 Tulndaria, Sertularia, and Medusa. The name was applied 

 by Huxley in 1856 to a group corresponding to that termed 

 Hydromedusx by Vogt (1851) and Htdusx by Leuckart 

 (1853), and embracing the forms placed by Gegenbaur in 

 his Elements of Ccmparative Anatomy (1878) in four classes, 

 viz., Hydromedusx, Calycozoa, Thecomedusx, and Medusx. 

 Our knowledge of the structure and life-history of the 

 Hydrozoa, many of which, on account of their delicacy and 

 oceanic habits, are excessively difficult to obtain in a state 

 fit for investigation, has greatly extended within the last 

 five years. Whilst in the two decades preceding this period 

 the admirable researches of Huxley, Gegenbaur, Agassiz, 

 and Allman had brought to light and systematized a vast 

 mass of information with regard to these organisms, the 

 later observations of Claus, the Hertwigs, Haeckel, and 

 Metschnikoff, have corrected, extended, and added to 

 their history, especially in respect of embryological and 

 histological detail. An epitome of the present condition 

 of our knowledge of the group is afforded by the subjoined 

 tabular classification of its families, orders, and sub-classes. 



The definition and synonymy of the divisions recognized 



will be entered into, after a sketch has been given of the 

 common structural features of typical Hydrozoa. 



CLASS HYDROZOA. 



Sub-Class I. Scyphomedusse (syn. Ephyromedmae). 



Order 1. LCCEENABLK. Example*. 



Fam. 1. Eleutherocarpid* .......... { 



Order 2. DISCOMEDUS.S (Haeckel). 

 Sub-Order 1. Cuoostoma?. 

 Fam. 1. Protephyridje. 

 i. Nausithoid*. 

 *. EphyrellidK. 

 4. AtoUids. 

 i. Cyclorchidas. 

 Sub-Order 2. Semostomae. 



Kausithoe. 



Fam. 1. 



2. Cvanjeidae ............... 



\ 3. Sthenonidje 

 4. Anrelidn 

 Sub-Order 3. Rhizostnmse. 



Fam. 1. TetragamelUe 



2. Monogamelix ... 



Order 3. CoxoMEcrsiai (Haeckel). 



Fam. 1. Charybdeidse 



2. Bursarida 1 . 

 3. Chiropsahnidff. 

 Order 4. PEEOMEDUS^ (Haeckel). 

 Fam. 1. Periphyllidw. 

 ,, 2. Pericn-ptidie. 



Sthenonhu 



Aurelia (figs. 26-31). 



ICephea. 

 Cassiopeia. 



Rhizostoma (fig. 24, a). 



Cliarybdaea (figs. 20-23). 



