TYPE CCELENTERA. 87 



tulariaand Ilaleciuni, and in the Plumularidae, e.g. Aglaoplieuia, 

 it would appear at first sight that no medusa generation oc- 

 curred. This, however, is not strictly accurate, the appear- 

 ance depending on the medusa-buds in these forms never 

 becoming free-swimming but remaining in a more or less 

 undeveloped condition, the ova and spermatozoa becoming 

 mature notwithstanding the imperfect development of the 

 medusae. Alternation of generations, however, exists in these 

 forms just as much as in the typical Campanularians, for, no 

 matter how degenerate the buds of the gonopolyps may be, 

 they must still be regarded as the medusa generation. 



5. Order Tubulariae or Anthomedusae. 



In this order, as in the preceding one, a well-marked 

 alternation of polyp and medusa generations occurs. The 

 polyps are united to form colonies, the individual hydrauths 

 being constructed on the Tubulariau type, i.e. the perisarc 

 ceases at the base of each hydrauth so that there is no 

 hydrotheca. The tentacles show a greater variet}- of form 

 and structure than in the Campauulariau forms, and, though 

 sometimes filiform and arranged in a single cycle at the base 

 of the hypostorne (Margelis), are yet in other forms scattered 

 irregularly over the surface of the hypostome (Clavd), or may 

 be club-shaped (Coryne), or in addition to scattered club- 

 shaped tentacles a circle of filiform ones may occur at the 

 base of the hydranth as in Pennaria. A division of labor is 

 not the rule as in the Campanularians, but nevertheless this 

 phenomenon is in some cases carried to a much greater ex- 

 tent than in that group. In some forms special gonopolyps 

 are present from which all the medusae arise, but more fre- 

 quently any of the hydranths may produce these structures. 

 The gouopolyps when they occur are never enclosed within a 

 gonotheca, and hence the term gymnoblastic, frequently applied 

 to the polyps of this order. In the genus Hydractinia (Fig. 

 44) a complicated division of labor occurs. The various 

 hydranths composing the colony arise from a flat expansion 

 common to all and formed by the fusion of an original net- 

 work of ccenosarcal tubes, and on the surface of which numer- 



