On the Striictnre and At'finities of Maestra parasites Krohu etc. 43 



nematocysts lills the uarrower parts. The nuclei of the cnidobhists 

 are easily distinguishal)le from those of the interstitial cells because 

 they are flattened by close appositiou to the nematocysts. Each 

 cuidoblast is provided with a cnidocil ; a drawiug of a transverse 

 section across a cnidact is shovvn in hg. 9. 



The nematocysts when mature, respond readily to mechanical 

 and chemical Stimuli. The discharged threads are very thin at their 

 free extremities, but are stouter at the proximal ends. The bases 

 of the threads are armed with three or four retlected barbs (fig. 8). 



The periphery of the umbrella is not so well supplied with 

 thrcad cells as the streaks on the exumbrella. Cnidoblasts also occur 

 upon the tentacles, especially on their bulbous bases. 



The Ectoderm of the Tentacles. 



The ectoderm cells are like the epithelio-muscular cells of many 

 other Hydrozoa. Maceration preparations made by the classical 

 method of the Hertwigs, show that the epithelio-muscular cells are 

 columnar in shape, with nuclei near their inner ends, vacuolated 

 near their outer ends and provided with very fine muscular pro- 

 cesses which in the living animai form a layer immediately outside 

 the mesogloea. 



Along the ab ral margin of certain of the tentacles is a row 

 of cnidophors, stalked ovoid bodies shaped like the conidiophors 

 of Uredineous Fungi (fig. 16). The knobs of these peculiar organs 

 are composed of an outer envelope of numerous small ectoderm 

 cells surrounding four cnidoblasts (fig. 17). The stalks are wonder- 

 fully extensile like those of the somewhat similar organs among 

 the Siphonophora. I could not ascertain the structure of the stalk 

 to my satisfaction even with a 7i2 ^"^^^ oil immersion lens, and 

 the question of its being uni-or multi-cellular is left undecided. 

 Allman described the similar peduncles of Gemmcma as being "of 

 true sarcode identical with that of Rhizopoda". When at full ex- 

 tension the stalks were four or five times as long as the terminal 

 knob, but when contracted the lengths of stalk and knob became 

 approximately equal. 



The cnidophor stalks like those of Gemmaria implexa Allman, 

 under a low power, show au irregulär contour. Under a high 

 magnitìcation the stalk is seen to be beset with minute processes 

 whieh sometimes present a curious foliated appearance (fig. 1 7 a, f). 



