42 ESSENTIALS OF ZOOLOGY 



chitinous sheath. The reproductive zooid or gonophore when 

 fully developed is enclosed, therefore, in a chitinous gonotheca ; 

 it is much larger than the hydrotheca, is elongated, and 

 terminates in a cone-like projection. An opening is formed 

 at the summit for the escape of the medusae when these are 

 developed. All gonophores produced by marine hydroids do 

 not liberate medusae : in many cases the gonophores liberate 

 only sex cells or larvae. But in the case of Obelia and 

 many other similar forms medusae are developed which 

 become separated, and during a period of free existence ripen 

 and liberate the sex cells or larvae. 



The gonophore of Obelia, as has been seen, consists of a 

 case or gonotheca containing a blind outgrowth of the coeno- 

 sarc ; the latter is made up of ectoderm and endoderm, and 

 contains a diverticulum of the gastric cavity. This part of 

 the gonophore is called the blastostyle. The blastostyle 

 contracts, and at the same time bud-like outgrowths appear 

 in succession along its walls. How these are gradually con- 

 verted into medusoids will be plain from an inspection of 

 fig. 18. The medusoids are liberated and commence a pelagic 

 life, during which the sex cells are ripened in rounded clusters 

 on the radial canals. When these in turn are liberated the 

 medusoid degenerates. 



The medusoid is umbrella-shaped ; the outer convex surface 

 is the umbrella surface, and the inner the sub-umbrella surface. 

 The margin bears a large and variable number of tentacles, 

 but usually 'twenty- four. From the roof of the sub-umbrella 

 cavity a long stalk or manubrium projects, and it bears the 

 mouth at the free end. The mouth leads into a gastric 

 cavity, which is resolved into a stomach at the top of the 

 manubrium, and four canals which radiate to the margin of 

 the bell, where they are put into communication by a circular 

 canal. This gastric system is lined throughout by endo- 

 derm. The external covering is of ectoderm, and the two 

 layers merge at the mouth. Between the two layers the 

 mesogloea forms a significant layer of a gelatinous nature. 

 The margin of the umbrella presents a circular nerve 

 associated with the ectoderm, and at intervals marginal 

 sense organs, of which in Obelia there are eight at the bases 



