\H\ TIIK AMKKU'AN MONTH l.V [April, 



phiccs : these are tlie j^oiiatls — male (ir spermai ies. ami female or 

 ovaries — ami the sexes of the Potlocoiyne are se})aiate. 'The 

 eii^s are set tree in the water, where, if they are tertili/.etl. tliey 

 tlevelop throin:l> sta>;es similar to those of llytlra. ami a smaller 

 h\ilra-l"orm eml>ryi> linally t"i>rms, which attaches itself and forms a 

 new coloiiv l>y the asexual process of InuUlinj;. Where twt) 

 (jiiite unlike forms, like the hvdrozooidand jjono/.ooid, occur in an 

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

 reprotluction alternate, it is called ixw a/tcr nation of octicrcif ion Ahc 

 ej;}4s not l>ein«j produceil ilirectly l>y the individuals which jjrew 

 from ej^j^s. Init from a ijeneration of individuals produced asexuallv 

 tVom those produced from the ej;«;. 



1 1^ niJAi riMA kc iiina ia.* (fiij. 19). 



This marine hvdroid ft)rms a pinkish film on the shells of 

 snails, r. ;[,'"., Lunalia tenanted by hermit-crahs. It is a poly- 

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

 zooids or members ; these arc first the stolon or spreadinjj net- 

 work of tubes of ectoderm anil endoderm. which, by buddin<i;,give 

 rise to the entire colony ; then there are the feetlin^ zooids 

 or nutritive polyps, which compare closely with Hydra or 

 Podocoryne. being tubular, terminatetl 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 l>earing on its sides numerous large sj^herical bodies which 

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

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

 dus;e. 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 ofVer 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. 



TlUUI.ARIA DIVISA.t 



This is a colony of salt-water hvdroids. common in many places 

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

 jects. It is notable for the large size attained l)y the zooids, they 

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



•I-- I I'ritt. ix., p. 561. 



Ag;. studies, p. 73. 



Pa'. .y, p. 56. 



Explanation OP THR FiGi'HP. OF HvDBACTiNiA. 

 Fig. 19. From nature : part of colony scrnped frcm I.unaiia, showing the rhizoid or stolon 

 R/ . ..:r! ii- i,r..n' hiiv. and thrrc forms of persons ; nut, the nutritious person ; dac, the 

 I killing member, and med, the medusa producing poison ; in this case 

 ■ ry, and do not become free. 

 , p. 72. Riverside Natural History, vol, i, p. 80. 



