96 THE AMERICAN MONTHLY [April, 



places : these are the gonads — male or spermaries, and female or 

 ovaries — and the sexes of the Podocoryne are separate. The 

 eggs are set free in the water, where, if they are fertilized, they 

 develop through stages similar to those of Hydra, and a smaller 

 hydra-form embryo finally forms, which attaches itself and forms a 

 new colony by the asexual process of budding. Where two 

 quite unlike forms, like the hydrozooidand gonozooid, occur in an 

 animal life history, and where the sexual and asexual process of 

 reproduction alternate, it is called an alternatio7i of ge7teratio7i^ the 

 eggs not being produced directly *by the individuals which grew 

 from eggs, but from a generation of individuals produced asexually 

 from those produced from the Q^%. 



Hydractinia echinata.* (fig. 19). 



This marine hydroid forms a pinkish film on the shells of 

 ^x\?i\\^^ e. g.^ Lutiatia tenanted by hermit-crabs. It is a poly- 

 morphic colony — that is, a colony composed of several kinds of 

 zooids or members ; these are first the stolon or spreading net- 

 work of tubes of ectoderm and endoderm, which, by budding, give 

 rise to the entire colony; then there are the feeding zooids 

 or nutritive polyps, which compare closely with Hydra or 

 Podocoryne, being tubular, terminated with tentacles surrounding 

 a manubrium , also the dactylozooid or protective person, which 

 has no mouth or tentacles, but is very mobile and sensitive, and, 

 moreover, is armed with a very formidable array of nettle-cells, 

 and also the generative zooid, a stem terminated with nettle-cells 

 but bearing on its sides numerous large spherical bodies which 

 contain ova ; these are believed to be degenerate medusas, com- 

 parable with the gonozooids of Podocoryne. There are no me- 

 dusas, but the eggs are set free in the water from the generative 

 zooid ; there is thus in this case no true alternation of gener- 

 ations. Hydractinia has a skeleton surrounding the stolon at the 

 base of the colony and projecting in the form of hard spiny prom- 

 inences which must offer perfect protection to the stolon and con- 

 siderable protection to the zooids when they are contracted to 

 their utmost. In this case we can see that polymorphism is 

 carried even farther than in Podocoryne. 



TUBULARIA DIVJSA.f 



This is a colony of salt-water hydroids, common in many places 

 on our coast, growing on wharf piles and other submerged ob- 

 jects. It is notable for the large size attained by the zooids, they 

 being often as much as \ inch across. There are distinct stems, 



♦Lankester, E. Britt. ix., p. 561. 



Agassiz, Seaside Studies, p. 73. 



Packard, Zoology, p. 56. 



Explanation of the Figuke of Hydractinia. 



Fig. 19. From nature ; part of colony scraped from Lunatia, showing the rhizoid or stolon 

 (Rz;, and its branchings and three forms of persons : nut, the nutritious person ; dac, the 

 dactylozoid or sensory and killing member, and med, the medusa producing poison ; in this case 

 the medusa; are rudimentary, and do not become free. 



t Agassiz, Seaside Stud., p. 72. Riverside Natural History, vol. i, p. 80. 



