iv PHYLUM CGELENTERATA 103 



of the medusa (Fig. 45), 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, 

 containing calcareous bodies or lithites. These simple free- 

 swimming medusiform Hydrozoa (Trachylina) develop ova 

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

 but the latter, instead of becoming fixed and developing into 

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

 medusae like those from which they originated. The fixed 

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

 nation 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 gonothecae for the 

 protection of the polypes and blastostyles respectively ; 

 but in others (Fig. 46) these protecting structures are 

 absent. The polypes resemble those of Obelia in all es- 

 sential respects, but differ in the number and arrangement 

 of the tentacles and other minor points. In many medusae 

 are developed from blastostyles as in Obelia, and when 

 fully formed become free. The shape of the medusa 

 differs in different forms, more particularly as regards the 

 umbrella. There is always a manubrium, with gastric 

 cavity, and a marginal and four radial canals ; and a velum 

 is universally present. But lithocysts are not present in all, 

 their place being taken by specks of red or black pigment 

 the ocelli or rudimentary eyes at the bases of the ten- 

 tacles. The number and arrangement of the tentacles is 



