82 INVEBTEBBATB MOEPHOLOGT. 



development of tlie nervous system than is found in the 

 polyps obtains. The sense-organs vary both in structure and 

 function in different medusae, in some being spots of pigment 

 sometimes provided with a refractive lens and functioning as 

 light-percipient structures. Such medusae (Fig. 45) are 

 termed ocellate, or, since the eyes are on the surface of the 

 bell and are not enclosed within a chamber formed by the 

 growth around them of the adjoining tissues, they are some- 

 times termed gymnophthalmatous. In other forms again the 

 sense-organs consist of a group of cells containing in their 

 interior crystals of carbonate of lime and having a somewhat 

 vesicular appearance, and in the neighborhood of these are 

 sensory cells with long stiff cilia to which are imparted any 

 vibrations which the crystals may manifest. Such crystal- 

 containing cells are known as otocysts, on the supposition that 

 they form auditory organs, but it seems more probable 

 that their function is that of equilibrium organs, informing 

 the medusae of their position. Medusae possessing such 

 organs (Fig. 43) are termed, to distinguish them from ocellate 

 forms, vesiculate medusae. 



The ectoderm which covers the outer surface of the bell is 

 much flattened, but near the rim it becomes columnar and 

 is ciliated, some of the cells assuming the form of sensory 

 cells similar in appearance to those of the polyps, their 

 slender nerve-filaments forming a delicate plexus in the 

 deeper layers of the ectoderm, in which are here and there 

 imbedded stellate ganglion-cells. A special nerve-ring sur- 

 rounds the bell at its margin. The ectoderm-cells which 

 cover both surfaces of the velum and the subumbrellar surface 

 are of the epithelio-muscular and muscular types, the mus- 

 cle-fibres being for the most part arranged circularlr. In 

 the deeper layers of the subumbrellar ectoderm numerous 

 ganglion-cells with long prolongations occur forming a net- 

 work immediately external to the muscle-processes, and at 

 certain definite regions collections of reproductive cells occur. 



The mesogloea of the subumbrellar surface and of the 

 velum is thin, resembling that of the polyps, but in the convex 

 portion of the bell it is very thick and gelatinous in texture, and 

 may in some cases contain cells. The endoderm is throughout 



