HYDROIDS. 



85 



gonotheca, but still remains attached to the blastostyle by a slender peduncle ; this 

 zooid is now seen to be sexual, and contains within the walls of its proboscis, the 

 sexual elements ; 

 the outline is nearly 

 spherical, being cut 

 off at the farther 

 end where there is 

 an opening into the 

 cavity of the zooid ; 

 about this opening 

 is a wreath of ten- 

 tacles, and pendent 

 in the cavity of the 

 bell is a proboscis 

 destitute of a 

 mouth; the cavity 

 of the blastostyle is directly contm- 

 uous with a central cavity in this 

 meconidium, as this kind of zooid is 

 termed, and from this central cavity 

 four radial canals pass out to four 

 equidistant points on the edge or rim 

 of the bell, where they all join a cir- 

 cular canal ; these meconidia never be- 

 come free, but after discharging their 

 contents, they die and disintegrate. 

 The fertilized eggs develop into cili- 

 ated planulae which finally form col- 

 onies of Gonothyrea hyalitia. 



These meconidse which are evi- 

 dently medusse that never become 

 free, are of great interest, being, in all 

 probability, degenerate forms. 



Another large family, the Sbetu- 

 LAEiD^, belonging to this same group, 

 is represented in America by a beauti- 

 ful species, Sertularia argentea, so- 

 called from its light silvery color. 

 The colonies are often a foot in height, 

 and the shoots usually grow in clusters ; 

 the branches have a subverticillate ar- 

 rangement, giving the colony an arbor- 

 escent appearance. If a small portion 

 of a colony be examined with a magni- 

 fier one discovers very peculiar hydro- 

 thecas, which are very differently ar- 

 ranged from any described above ; they are nearly tubular, somewhat narrowed at the 

 top, with pointed lips, and are either free or set into the sides of the stems and branches. 



Fig. 77 Gonotlieoa with meconidia of Gcmotltyrea; b, blasto- 

 style; c7, gonopliores in various stages of development; 

 g, meconidia; m, ovum; o, embryos. 



