248 DESCRIPTIONS OF PREPARATIONS. 



The lamina, which lies externally to the endoderm, i.e. on the convex side of the 

 bell, is very much thickened, and forms the- umbrella (z/.), the substance of which 

 is watery, jelly-like, and as a rule traversed by elastic fibres (e.f.). The umbrella is 

 thickest in the region of the atrium, thinnest at the apparent edge of the bell, close 

 to the circumferential canal. The lamina placed internally to the endoderm, i.e. 

 on the concave surface of the bell, is very thin, like that of the manubrium and 

 tentacles. It forms the sub-umbrella (s.u.). The umbrella and sub-umbrella pass 

 round the circumferential canal, and unite to form the simple supporting lamina of 

 the velum. The ectoderm varies in character in different regions. On the convex 

 surface of the bell the cells are flat, except to the outer side of the circumferential 

 canal, where there is a ridge of columnar ciliated cells, some of which send pro- 

 cesses to the outer nerve-ring (.), which lies at their base. The cells of the outer 

 surface of the velum are cubical. On the concave surface of the bell they are dis- 

 posed usually in a single layer, and develope at their bases, as indicated by thicker 

 dashes, circularly disposed muscle-fibres. Similar fibres exist on the inner surface 

 of the velum, but they are wanting, however, to the inner side of the circumferential 

 canal, where the inner nerve-ring ('.) lies. Scattered ganglion cells in connection 

 with this ring lie in the ectoderm of the sub-umbrella, and supply the muscle-fibres. 



The ectoderm of the manubrium developes (i) longitudinally disposed muscle- 

 fibres, indicated by thicker dashes, and (2) at the oral margin nematocysts or thread- 

 cells, which are situated on the oral tentacles when these structures are present. 



Tentacles (A: T.) usually spring from the margin of the bell at the ends of the 

 radial canals, sometimes also from the circumferential canal in the inter-radial 

 spaces. They are either tubular and lined by ciliated endoderm cells, or solid with 

 a core of vacuolated endoderm cells arranged in a single row. Their ectoderm cells 

 develope longitudinally arranged muscle-fibres, and nematocysts. 



A section taken inter-radially (Fig. n, B.) agrees in most respects with a 

 section taken in the plane of a radial canal (Fig. n, A.). But in the place of a 

 radial canal it shows a single layer of cubical endoderm cells, forming the so-called 

 gastral lamella (B : "./.), continuous with the edges of the atrium and of the circum- 

 ferential canal respectively. In this lamella new radial canals are sometimes 

 developed during the growth of the organism, e.g. in Aequorea. 



The eye-spots in an Ocellate Medusa are generally placed .at the base of a 

 tentacle on its outer side, sometimes, however, on the inner. The eye has retina 

 cells, supporting cells, basal ganglion cells, and sometimes a cuticular lens. The 

 auditory vesicle in a vesiculate Medusa is developed close to the inner nerve-ring, 

 from a pit of the inner surface of the velum, which projects towards its outer surface. 

 It has otolith cells and sense-cells, derived from the ectoderm of the inner surface 

 of the velum. 



A Medusa with a velum is said to be Craspedote. Such Medusae characterize 

 the Craspedota, one of the two main sub-divisions of the class Hydrozoa. The 

 other division is known as Acraspeda. 



The gonads or generative organs are situated on the manubrium in an ocellate 

 Medusa ; at the base of the manubrium or on the course of a radial canal in a vesi- 

 culate Medusa. The spermatozoa and ova often arise and invariably ripen in the 

 ectoderm, though they may take their origin in the coenosarc or the blastostyle, 

 and by what is apparently an abbreviation of development, from the endoderm. 



