SECT. IV PHYLUM OELENTERATA 101 



tome and a circlet of from six to eight tentacles surrounding 

 the mouth. It is ordinarily attached, by virtue of a sticky 

 secretion from the proximal end, to weeds, etc., but is capable 

 of detaching itself and moving from place to place after the 

 manner of a looping caterpillar. The tentacles are hollow, 

 and communicate freely with the enteron. There is no 

 perisarc. Buds are produced which develop into Hydras ; 

 but these are always detached sooner or later, so that a 

 permanent colony is never formed. There are no special 

 reproductive zooids, but simple ovaries {ovy) and testes 

 {spy) are developed, the former nearer the proximal, the 

 latter nearer the distal end of the body. 



In nearly all the remaining Hydrozoa that do not form 

 colonies the form assumed is not that of the polype, but that 

 of the medusa (Fig. 46), a polype stage never being 

 developed, and the animal resembling in all essential 

 respects the medusae of Obelia ; the chief difference of 

 importance being the presence of sense-organs in the form 

 of hollow, club-shaped appendages, the tentaculocysts, con- 

 taining calcareous bodies of lithites. These simple free- 

 swimming medusiform Hydrozoa {Trachylince) develop ova 

 and sperms which give rise to free-swimming ciliated larva; ; 

 but the latter, instead of becoming fixed and developing into 

 plant-like colonies, remain free, and develop directly into 

 medusa; like those from which they originated. The fixed 

 zoophyte stage is thus absent in the life-history, and an 

 alternation of generations is not recognisable. 



In the colonial Hydrozoa, which constitute the great 

 majority of the class, the colony in most instances resem- 

 bles that of Obelia in being a fixed structure consisting of a 

 slender branching stem, covered over by perisarc, and bear- 

 ing zooids and blastostyles. In many the perisarc is 

 produced to form hydrothecae and gonotheca; for the 



