HYDEOZOA 



THE Hydrozoa form one of the three classes into 

 which the Coelentera nematophora (distinguished from 

 the Coelentera porifera, or Sponges) have been divided. 

 It results from observations made by Ernst Haeckel that 

 the Ctenophora 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 



Scyphomediisse from the Deep Sea. (After Haeckel, Challenger Reports, vol. iv. 1882). 



A. Periphplla mirabilis, Haeck,,one of the Peromedusse, one-third the natural size, a, one of the four intei-radial tentaculocysfa (sensory organs) sunk 



hetween its lappets ; &, one of the sixteen subradial coronal lohes^ The twelve tentacles (four peiTadial, eight adradial) are seen. 



B. Perradial section through Jjucemariabathyphila, Haeclc, nat. size, o, perradial gastral pouch ; 6, gastral axial cavity; c, ovary (four); d, gastral filaments; 



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

 The eight principal tentacles (four pen'adial and four inteiTadial) are not in this species converted into adhesive anchors as in L. auriciila, but are 

 altogether suppressed. 



groups or grades, for which the names Polypomorpha and 

 Ctenophora are proposed. 



The Hydrozoa correspond to the Linnsean genera Hydra, 

 Tubvlaria, Sertularia, and Medusa. The name was applied 

 by Huxley in 1856 to a group corresponding to that termed 

 Hydromedusx by Vogt (1851) and Medusse. by Leuckart 

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

 his Elements ofCcmparative Anatomy (1878) in four classes, 

 viz., Hydromedusai, Calycozoa, Thecomedusx, and Medusse. 

 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 

 Metschnikofi^, 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. Scyphomednsse (syn. Ephyromedusse). 



Order 1. LncEKNARi^ai. Examples. 



Fam. 1. Elcutherocarpid. { JiXr/stil^"^- ">■ 



„ 2.Cleisi»ca,Tid. -iSTnS^""^- 



Order 2. DrscoMEDDS.ffl (Haeckel). 

 Sub-Order 1. CubostomiE. 

 Fam. 1. Protephyridse. 



„ 2. Nausithoidffl. Nausithoc. 



,, 3. Ephyrellidse. 

 ,, 4. AtoUidffi. 

 ,, 5. Cyclorchidae. 

 Sub-Order 2. Semostomse. 



T- « 1 T> 1 „',7« S Chrysaora (fig. 24, 6). 



Fam. 1. PelagidBB j Pelagia. 



„ 2. Cyanseidse CyanEea. 



„ 3. Sthenonidae Stlienonia. 



„ 4. AurelidEB Aurelia (figs. 26-31). 



Sub-Order 3. Rhizostomje. 



Fam. 1. Tetragameli£E { ca^gfopeia. 



„ ,, ,, ( Rhizostoma (fig. 24, a). 



„ 2. Monogameliffi [ Crambessa. 



Order 3. Cokomedusjb (Haeckel). 



Fam. 1. Charybdeidse Cliaiybdaja (figs. 20-23). 



„ 2. BursaridsB. 

 ,, 3. Chiropsalmidas. 

 Oixier 4. PEKOMEDTjs.ffi (Haeckel). 

 Fam. 1. Periphyllidse. 

 , , 2. Periciyptidas. 



H 



