190 The Ciliate Infusoria. By G. J. AUman. 
orifices. This is regarded by Balbiani as a permanent digestive 
canal. The post-oral or pharyngeal portion of this tube possesses 
a very remarkable feature, namely, a longitudinal striation caused 
by rigid rod-like filaments which are developed in its walls, and 
which can be 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 eminently voracious and 
carnivorous, and when in pursuit of other living Infusoria, such as 
Paramcecium, 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 (Fig. 5 a). By the retraction of this tongue the prey 
is now gradually withdrawn towards the mouth, engulphed in the 
distended pharynx (Fig. 5 6), and pushed deeper and deeper into the 
axial canal, where it is digested, and the effete matter ultimately 
expelled through the anus. 
From all this Balbiani concludes against the unicellular doc- 
trine. 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 liquid of the 
Polyzoa, and of many other lower animals. He is not, however, 
disposed to make too broad a generalization, and to insist on the 
presence of an alimentary canal distinct from a body cavity in all 
the other Infusoria. Here, however, he falls in with the views of 
Claparede and Lachmann and of Greeff, and maintains that as a 
rule the digestive and body cavity in the Infusoria are confounded 
into a single gastro-vascular system. 
Independently, however, of the untenableness of the conception 
of a united digestive and body cavity, it does not appear to me that 
Balbiani makes out any case against the unicellularity of the 
Infusoria. He admits that except in the pharyngeal and anal 
portion there is no evidence of a differentiated wall in his so-called 
digestive canal, and even though it be conceded that the middle 
portion of this canal constitutes a permanent cavity in the 
parenchyma, it would not differ essentially from other lacunae per- 
manently present in the protoplasm of many undoubtedly unicellular 
organisms. It has been already remarked that a communication 
between these lacunae and the external medium is paralleled in 
many simple cells, and these external communications in Didinium 
present no feature essentially different. 
The pharynx appears to be bounded by an inflection of the 
