« LIU>'EA>' SOCIKTY OF LOXDON. XXXI 



and a posterior wreath of cilia, has one end continued into a pro- 

 hoscis-hke projection, which carries tlie oral orifice on its summit, 

 while an anal orifice is situated on the point diametrically opposite 

 to this. There is a very distinct cuticle, though the rest of the cor- 

 tical layer is very thin and can scarcely be optically distinguished 

 from the internal parenchyma, which exhibits manifest currents of 

 rotation. These flow in a continuous sheet along the walls from the 

 anal towards the oral side, and on arriving at the mouth, turn in 

 towards the axis, and then flow backwards along this until they 

 complete the circuit by once more reaching the anal side of the body. 

 No trichocysts are developed in the walls of the body. The contrac- 

 tile vesicle is large and is situated near the anal end ; it presents very 

 distinct pulsations, and Balbiani is disposed to believe in a com- 

 munication between it and the exterior. 



During the act of digestion a tubular cavity can be seen running 

 through the axis of the body and connecting the oral and anal ori- 

 fices. This is regarded by Balbiani as a permanent digestive canal. 

 The postoral or pharyngeal portion of this tube possesses a very re- 

 markable feature — namely, a longitudiual striation caused by rigid, 

 rod-like filaments, which are developed in its walls, and which can 

 b? easily detached and isolated by pressure, or by the action of 

 acetic acid. They then resemble some common forms of the 

 raphides developed in the cells of plants. The function of these 

 rods becomes apparent when the animal is observed in the act of 

 capturing its prey. The Didinium is eminentty voracious and car- 

 nivorous, and when in pursuit of other living Infusoria, such as 

 Paramecium, the prey may be seen to become suddenly paralyzed 

 on its approach. A careful examination will then show that the 

 Didinium has projected against it some of its pharyngeal rods; and to 

 the action of these bodies the arrest of motion is attributed. A 

 curious cylindrical tongue-like organ is now projected from the 

 mouth towards the arrested prey, to which it becomes attached by 

 its extremity. By the retraction of this tongue, the prey is now 

 gradually withdrawn towards the mouth, engulfed in the distended 

 pharynx and pushed deeper and deeper into the axial canal, where 

 it is digested, and the eff"ete matter ultimately expelled through the 

 anus. 



From ail this Balbiani concludes against the unicellular doctrine. 

 He sees in the axial cavity a permanent alimentary canal, and in the 

 surrounding parenchyma a true perigastric space filled with a liquid 

 which corresponds with the perigastric Hquid of the Polyzoa and of 



