

HYDEOZOA. 



33 









tnit 



; 



m . 



lore 



other [ 



j 



lygm 







Transverse section of a tentacle shows it to con- 

 sist of ectodermal and endodermal layers, enelo- 



The 



sing a process of the somatic cavity, which^iu 

 very many cases, is wholly obliterated. " 

 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 appearance (fig. 19, b). 



Except in the Calycophoridce and Physophoridce, 

 the tentacles are very seldom branched. In some 

 * Hjjj 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 Physalia 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 

 ' , t: and the sac communicate with a canal which runs 



through the entire length of the tentacle. The 



01 side opposite the reniform enlargements is bor- 



o De dered by a wide muscular band, which connects 



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



udep:: form enlargements are, in other genera, replaced 



ire. S by lateral branches, some of which present three 



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



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

 de) sion, with 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 



