HYDROZOA 



HYDEOZOA form one of the three classes into 

 JL wliichthe Ccelentera 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 



B 



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



A. PeriphyUa mirabUis, Haeck., one of the Peromedusw, one-third the natural size, a, one of the four interradial tentaculocysts (sensoiy organs) sunk 

 between its lappets ; b, one of the sixteen subnulial coronal lobes. The twelve tentacles (four peiTadi.il, eight adradial) are M-en. 



13. Perradial section through Lucernarui bathyphila, Haeck., nat. size, or, perradial gastral pouch ; 6, gastral axial cavity ; c, ovary (fouv); d, gash-al filaments; 



e, peiTadial gastral pouch ; /, manubvium and mouth ; g, the bundles of tentacles (eight, adradial). 

 The eight principal tentacles (four perradial and four interradial) are not in this species converted into adhesive anchors as in L. auricula, hut arc 



altogether suppressed. 



groups or grades, for which the names Polypomorpha and 

 Ctenophora are proposed. 



The Hydrozoa correspond to the Linnajan genera Hydra, 

 Tuljidaria, Sertularia, and Medusa. The name was applied 

 by Huxley in 1856 to a group corresponding to that termed 

 Hydromedmx by Vogt (1851) and Medusse. by Leuckart 

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

 his Elements of C cm/par alive Anatomy (1878) in four classes, 

 viz., Hydromedmx, Calycozoa, Tliecomedusx, 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 Glaus, 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. Scyphomedusae (syn. Epliymmcdusw). 



Examples, 



f Lncernaria (fig. 19). 

 '( Halidystus. 

 < Craterolophus. 

 "i Manama. 



Order 1. LUCEBSASIS. 



Fam. 1. EleuthcrocarpMa 1 . 



2. Cleistocarpidze 



Order 2. DISCOMEDUS.E (Haeckel). 

 Sub-Order 1. Cubostoma'. 

 Fam. 1. Protcphyridse. 



2. Nausitholdse Nausithoc. 



,, 3. Ephyrellidaj. 

 ,, 4. Atollidaj. 

 ,, 5. Cyclorchida?. 

 Sub-Order 2. Semostoma;. 



I Chrvsnnrn (fig. 24, l>). 

 Fum. 1. Pelagidffi i l' C ll K la. 



, 2. Cyana?id!c Cyamra. 



, 3. Sthenonidse SOienonte. 



4. Aurclldn Aurelia (figs. 26-31). 



Sub-Order 3. Rhizostomre. 



Fain. 1. Tetragameliae 



2. Monogamelia? 



Order 3. COKOMEDCSJE (Haeckel). 



Fam. 1. Charybdeidse 



,, 2. liursarida 1 . 

 ,, 3. Chiropsalmid.T. 

 UnkT 4. I'i;unMKi'rp.E (Hiieckel). 

 Fam. 1. Pcriphyllidje. 

 ,, 2. Pericrj'pHdse. 



Cephca. 

 C'nssidpcia. 

 Hhizostoma (fig. 24, n). 

 CiamlKSBa. 



s. 20-2;i). 



