40 



AMERICAN HYDROIDS. 



endoderm of the stem and finally become aggregated under the points of insertion of little bnds 

 or stumps, which are the beginnings of the corbula'. These stumps make their appearance early 

 in the life of the colony, and remain quiescent in a rudimentary state during the growth of the 

 hydrocladia above them. Afterwards these stumps grow larger and are, filled with the sex cells. 

 According to Weismann's figure, a nematophore is born on a very young corbula stump. 



The later development has been studied by Alluian, ' who gives a careful description of the 

 development of the corbula- of Aylaophenia plumn. While at the Marine Biological Laboratory in 

 Plymouth, England, I had an excellent opportunity to study the same species and to sketch the 

 various stages of growth (figs. 107-lir>). The succession of events is as follows: 



112 



113 



HKVELOPMENT OP THE CORBULA OF AQLAOPHEN1A 1M.UMA. 



Figs. 107-llfi art' jirrail-ji-il in tln-ir prnpi-r sr<iiieiK'u 1" show the progressive ik-velnpment nf the eurlmla. 17, ^mmphon-; n, nematophore; 



s, septum of corbula leaf. 



First, the corbula stump produces a hydranth with hydrotheca and the three sarcostyles with 

 their nernatophores. I was unable to determine whether the hydrauth or sarcostyle made its 

 appearance first. The distal end of the growing twig in front of the hydrotheca appears to be an 

 unclosed tube slightly swollen near its extremity. About halfway between its end and the margin 

 of the hydrotheca the bud of the first corbula leaf appears in the form of a broad oval process from 

 the twig with an oval, apparently open end directed somewhat laterally. The second leaf appears 

 between the first and the open end of the twig and is directed toward the side opposite the first. 

 At this stage the bud of the first gonophore appears in front of the base of the first leaf. Other 

 leaves now appear in alternate succession. The six-leaved corbula (fig. 110) shows that the 



'A Monograph of the (iyniuolilastic or Tubulariiui Hyilroiils, Kay Society, London, 1871, p. 60. 



