390 EVENINGS AT THE MICPwOSCOPE. 



sisting between tlie poljparj and its associated organs, 

 all combine to form an object of the highest in- 

 terest, and indicate that, in this fixed Yet travellincr 

 zoopliyte, we have a type of structure transitional 

 between the dendritic IIydroid(B and the more higlily 

 organized AcalepJi. In the simplest acalephoid form, 

 such as tlie medusoid of Comjpanularia [or Laomedea,'] 

 (wliich is nothing more tlian an extension of the poly- 

 pary specially organized for independent and motile 

 life,) we have (as in Ilydractinid) an expanded poly- 

 pary, represented by the umbrella, and permeated 

 by vascular tubes from the confluence of which last 

 spring, at the centre, the tentacular polyps, various in 

 number; and between them the reproductive polyps, 

 represented by the sessile generative sacs." * 



You see here a jar, on the glass side of which 

 are traced a number of very fine white lines, barely 

 discernible by the unassisted eye. Eut by the aid of 

 the lens you see that each line is a long and slender 

 thread, which creeps along the glass, and at len^-th 

 starts out from it free for a sliort distance, and is then 

 terminated by a long club-sliaped body, which carries 

 at its extrem.ity four horizontally divergent organs, like 

 the arms of a turnstile. Tracing down the threads to 

 their lower extremities, you see that they are branches 

 of one thread, which creeps irregularly over a filamen- 

 tous sea- weed growing from a stone in the jar. The 

 Bea-weed had been in the vessel for several weeks, and 

 the water having been undisturbed, the knobbed 

 thread, Mhich was originally confined to the plant, 

 continued to grow, and coming into contact with the 

 glass spread upon it. Many other threads have ex- 



* Ibid., op. cit. 



