lIYDItOZOA. 



43 



I-'i.;. 4. 



Meilusii l)C'iiiiJ- unablo to swim, luit (jiily ('apaljlc of creepiu}^ about on (.a,<: >, 

 its peculiar tentacles. 



Free-swimuiin«; geuerative polyps or Medusiu occur only in some 

 Hydroifls ; in many species the geuerative polyps remain on the 

 colony and produce the eggs in this situation ; in such tiiSf« they 

 lose, to a greater or less extent, their Medusan strnctun.- and may l>e 

 reduced to a mere wart (Fig. 7 ). 



Generative polyps may arise from the stem as in Buwjaiiii'Ulm, 

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

 poly}) which has lost its mouth and tentacles, and is then termed a 

 blastostyle ; the latter, with its buds, may be naked ( Ifi/ilnirfuiui, 

 Fig. i>), or invested with a horny capsule {Sertularia, Fig. Vl uh). 



The Hydroida are divided into two 

 groups — Athecata and Thecaphora. 



Athecata (or Gymnohladed-Anlho- 

 mechisce). In this group the feeding 

 polyps cannot be withdrawn into horny 

 cups, and the generative i>olyps or 

 buds are not enclosed in horny capsules. 

 The ^ledusffi (Authomedusa') form the 

 eggs in the walls of the manubrium. 

 Tuhularia i/idivisa* 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 Ijud contains a degenerate Medusa, 



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

 attached situation. The fertilised egg develops iiuo a peculiar embryo, 

 whir-li becomes fixed and grows up into a long-stenuned polyp. 



Tuhularia larijnx* has branched annulated stems. The aj-punut 

 branches on T. indivisa arise from embryos settling (»n the oMer 



stems. 



The fine specimen of Peniuiriu ruroluiii* from .Nai-l-s^ hm'u^ .i 

 branching colony, the polyps of wlii.'h possess a ring of liliform 



Free generntivi- i>i)ly|> or 

 !Mi(lu8ii of Jiougahiril- 

 ha frutirosa. a. iimiiu- 

 briutn; b, nuUal cftimls; 

 c, vellum ; d, nuked eyi- 

 spots. (After Allmtui ) 



