276 CESTODES. 



provided anteriorly with a more or less highly developed apparatus 

 of hooks (Caryophyllceus, Amphiptyches). In such cases there is no 

 distinction between nurse and sexual animal, or head and joint ; the 

 life-history, which is in ordinary cases spread over two generations, 

 is here perfected in the same individual ; instead of alternation 

 of generations a simple metamorphosis is seen. 



Physiologically, the polyzootic tape-worm is a composite colony. 

 Sensation and movement, nutrition and excretion, are distributed 

 uniformly over all the joints, as if they were simply organs of one 

 individual and were not themselves individualised. In respect of 

 their functions they are, indeed, like organs, or "parts of a higher 

 unity," but the unity which they form and uphold by their combined 

 effort is, morphologically, not a single individual but a colony. 



It was obviously these indications of a physiological unity which 

 hampered the true conception of the structure of the Cestode, and 

 ever recalled the old opinion that the tape- worm, even in its joitned 

 form, was a single animal. It was seen that the movements passed 

 as waves from one joint to another, that considerable portions of the 

 worm could extend their length, or, by contracting, increase their 

 breadth. How was this possible, except on the supposition of a 

 common and single impulse ? A number of canals were seen 

 running continuously throughout the chain; did not this prove the 

 individual simplicity of the body ? Was it not, after all, more 

 natural that the nurse should be regarded as the "head," and the 

 divisions filled with eggs as the "joints ?" 



The physiological unity of a polymorphic community is not less 

 wonderful now than it ever was, but we have gradually become accus- 

 tomed to it. We see the same thing in every case where similar 

 conditions obtain. 1 Could one doubt that the bees of a hive form a 

 colony in spite of their separateness and various kinds, or that the 

 operations of the indisputably distinct individuals of such a colony 

 are not as mutually complemented and co-operative as if they were 

 directed by an individual will ? 



But no more need be said in favour of a theory which is suffi- 

 ciently justified by the facts of development. 



We have to distinguish, then, in the so-called "tape-worm" 

 two kinds of individuals the " head," originating the whole colony 

 (the "root" of Pallas, the "bulb" of Reirnarus), and the sub- 

 sequently produced "joints." At the first glance the latter differ 

 markedly from one another, the anterior joints near the head (the 

 " neck ") being small, and often indistinctly separated ; but all the 



1 See my treatise on Polymorphism, referred to above. 



