HYDROZOA. 
43 
Fig. 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 Bougainvillea , 
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 (By dr actinia, 
Fig. 9), or invested with a horny capsule ( Sertularia , Fig. 12 b b). 
The Hydroida are divided into two 
groups — Athecata and Thecaphora. 
Athecata (or Gymnoblastea-Antho- 
medusce). 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polyps cannot be withdrawn into horny 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 (Anthomedusae) form the 
eggs in the walls of the manubrium. 
Tubularia 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 egg develops into a peculiar embryo, 
which becomes fixed and grows up into a long-stemmed polyp. 
Tubularia larynx * has branched annulated stems. The apparent 
branches on T. indivisa arise from embryos settling on the older 
stems. 
The fine specimen of Pennaria cavohnii * from Naples shows a 
branching colony, the polyps of which possess a ring of filiform 
Free generative polyp or 
Medusa of Bougainvil- 
lea fruticosa. a , manu- 
brium ; b , radial canals ; 
c, vellum ; d, naked eye- 
spots. (After Allman.) 
