DEVELOPMENT OF THE PHTLACTOLJIJIATOUS POLTZOA. 493 



axis perpendicular to the walls of the oesophagus. They have in 

 their middle a large oval nucleus with clear strongly refringent 

 nucleolus. The nucleus divides the cell into an inner half and a 

 peripheral half. The peripheral half is clear, and has the appear- 

 ance of being empty and closed at its inner end by the nucleus, 

 while the inner half is filled with granular contents. The inner 

 half, moreover, is provided with a special cell-membrane, while 

 the peripheral half has no proper membrane. The peripheral 

 portions thus appear to constitute a system of lacunae in which the 

 wall of one forms a part of the wall of that abutting on it, and 

 which may be thus best compared with a honey-comb. 



Lying on the free end of each of these oesophageal cells may be 

 seen a small transparent vesicle, which at one time swells out into 

 a sphere, at another contracts into a short ovoid. These vesicles 

 seem to represent an internal epithelial layer of the oesophagus. 

 Where the oesophagus slightly dilates towards the mouth, they 

 are replaced by cells bearing long cilia, and here also the honey- 

 comb-like lacunae are absent. 



Tentacles. — The walls of the tentacles are composed of three 

 layers — an outer cellular layer, an inner epithelium, and an inter- 

 mediate homogeneous membrane. Muscular fibres also enter 

 into the composition of the tentacle ; but these do not form, as in 

 the body-walls and alimentary canals, a continuous layer. The 

 homogeneous membrane forms the proper foundation-layer of the 

 whole tentacular crown. It is in direct continuation with the 

 homogeneous membrane of the muscular layer of the oesophagus 

 and body-wall, and, like this, is easily coloured by carmine 

 solution. It forms also the foundation-layer of the interten- 

 tacular membrane, and is continued beyond the free margin 

 of this membrane along the opposed sides of the tentacles in 

 the form of a ridge, which, however, in the living animal, is 

 concealed beneath the outer cellular layer, beyond which it does 

 not project. 



The outer cellular layer of the tentacles is divided by this ridge 

 into two distinct portions. That which lies behind the ridge is 

 directly continued from the outer cellular layer of the body-wall 

 which passes uninterruptedly from the tentacular sheath upon 

 the back of the tentacles, where it presents the two elements 

 already described, the polygonal cells and the round cells. This 

 part of the cellular layer carries no cilia ; but, on the other hand, 

 fine, long, stiff bristles have been described by Nitsche as occur- 



