HYDROZOA. 



43 



Fk;. 4. 



Medusa being unable to swim, but only capable of creeping about on Case 3. 

 its peculiar tentacles. 



Free-swimming generative polyps or Medusae occur only in some 

 Hydroids ; in many species the generative polyps remain on the 

 colony and produce the eggs in this situation ; in such cases they 

 lose, to a greater or less extent, their Medusan structure and may be 

 reduced to a mere wart (Fig. 7). 



Generative polyps may arise from the stem as in Boiigainvillea, 

 but very commonly they arise from the sides of a degenerate feeding 

 polyp which has lost its mouth and tentacles, and is then termed a 

 blastostyle : the latter, with its buds, may be naked {Hijdractinia, 

 Fig. 9), or invested with a horny capsule {Sertularia, Fig. 12 b ft). 



The Hydroida are divided into two 

 groups — Athecata and Thecaphora. 



Athecata (or Gymnohlastea-Aiitho- 

 medusm). In this group the feeding 

 polyps cannot be withdrawn into horny 

 cups, and the generative polyps or 

 buds are not enclosed in horny capsules. 

 The Medusae (Anthomeduste) form the 

 eggs in the walls of the manubrium. 

 T'ubularia indivisa* or the " Tubular 

 Coralline like Oaten Pipes " of Ellis, forms 

 clusters of simple stems from six to 

 twelve inches in height, rising from a 

 twisted mass of roots. Each stem, in 

 life, is crowned with a red flower-like 

 polyp with two sets of filiform tentacles, 

 one set being near the mouth and the 

 other at the base ; festoons of stalked 

 generative polyps or buds hang down 

 from the base of the feeding polyp. 

 Each bud contains a degenerate Medusa, 



which never escapes to lead a free life, but produces the eggs in its 

 attached situation. The fertilised Qgg develops into a peculiar embryo, 

 which becomes fixed and grows up into a long-stemmed polyp. 



Tuhularia larynx* has branched annulated stems. The apparent 

 branches on T. indivisa arise from embryos settling on the older 

 stems. 



The fine specimen of Pennaria cacolinii * from Naples shows a 

 branching colony, the polyps of which possess a ring of filiform 



Free generative polyp or 

 Medusa of Boiigainvil- 

 lea fruticosa. a, manu- 

 brium; Z>, radial canals ; 

 c, vellum ; d, naked eye- 

 spots. (After AUman.) 



