INVERTEBRATA. 393 



special structure of ocelli. The eight adradial tentacles 

 have, in addition, projecting from the inner side of 

 the basal enlargements, small vesicles formed from 

 ectoderm and containing calcareous concretions. Such 

 organs were formerly regarded as rudimentary auditory 

 organs, but as our own semicircular canals are organs of 

 equilibrium, so experiment indicates that equilibration is 

 the principal function of these vesicular organs in the 

 medusa of Obelia and others of the lower animals. It is 

 better, therefore, to call them not otocysts, but statocysts. 

 The presence of these statocysts is characteristic of the 

 medusae called Leptomedusae, which are derived from 

 hydroids with hydrothecae. In the Anthomedusae there 

 are ocelli, but no statocysts, and the hydroids of these are 

 not provided with hydrothecae or gonothecae. In Obelia 

 there are never more than the eight statocysts on the eight 

 adradial tentacles, but the number of tentacles increases 

 considerably in older medusae, and each bears a pigment 

 spot. The tentacles are solid, a core of vacuolated endo- 

 derm cells extending along the axis of the tentacle as in 

 the hydroid. 



7. Development of Medusa. The mode in which the 

 medusa is developed as a bud on the blastostyle can now 

 be understood without difficulty (fig. 201). The first stage 

 of the bud is a simple small diverticulum of the cavity of 

 the blastostyle. This diverticulum grows longer and then 

 enlarges at the extremity so that it forms a little vesicle 

 enclosed by ectoderm, mesogloea, and endoderm, and con- 

 nected with the blastostyle by a narrow stalk. The cavity 

 of the subumbrella is now developed in a peculiar way 

 which would not be expected. The cavity is not formed 

 by the simple moulding of the vesicle to the shape of 

 a medusa, but the distal ectoderm separates into two 

 layers ; the inner layer acquires a cavity, and this little 

 sac, which may be called the bell-rudiment, enlarges 

 and acquires the shape of the subumbrellar cavity with the 

 manubrium in its centre. The cavity therefore, although 

 lined throughout by ectoderm, is at first closed by a layer 

 of ectoderm which extends from one margin of the 



