HTDROZOA. 33 



Transverse section of a tentacle shows it to con- 

 sist of ectodermal and endodermal layers, enclo- 

 sing a process of the somatic cavity, which, in 

 very many cases, is wholly obliterated. The 

 ectoderm is often found to exhibit muscular fibres, 

 and is always provided with numerous thread- 

 cells, which may accumulate near its surface in 

 minute rounded papilla?, imparting to the tentacle 

 a roughened apjDearance (fig. 19, h). 



Except in the Calycophoridcti and PhysoijJioridce, 

 the tentacles are very seldom branched. In some 

 genera of these orders the tentacles are as simple in 

 structure as those already described, but, in others, 

 they attain a much greater complexity. Each 

 tentacle of Fhysalia has a sac-like expansion at its 

 base, while numerous reniform enlargements, each 

 well packed with thread-cells, are disposed trans- 

 versely along its sides {fig. 11, d). Both these 

 and the sac communicate with a canal which runs 

 through the entire length of the tentacle. The 

 side opposite the reniform enlargements is bor- 

 dered by a wide muscular band, which connects 

 itself, above, with the basal dilatation. The reni- 

 form enlargements are, in other genera, replaced 

 by lateral branches, some of which present three 

 well-defined regions : a ' pedicle,' or proximal 

 slender portion; a ^ sacculus,' or "median divi- 

 sion, vrith one wall much thicker than the other, 

 containing numerous elongated thread-cells, ar- 

 ranged in transverse rows perpendicularly to the 

 wall, and flanked on each side by a longitudinal 

 series of larger oval thread-cells ; " and, finally, 

 a ' filament,' or '• terminal cylindrical thread, full 

 of large rounded thread-cells." Such, according 



D 



