NUMBER AND DEVELOPMENT OF THE SEGMENTS. 383 



The elongation and segmentation of the cervical portion 1 is as a 

 rule perceptible after a very short time. In Tcenia serrata I have 

 usually been able to distinguish with the naked eye, within forty-eight 

 hours of the feeding process, a chain of from twelve 

 to eighteen joints. In other cases, however, not a 

 trace of the segmentation was visible even after four 

 days. Sometimes one finds, even in the second or 

 third week, only a few straggling joints in a tape- 

 worm perhaps a foot long. 



As to the ways and means by which the indi- 

 vidual joints gradually grow and become sexually 

 mature, we shall have much to say in our detailed 

 survey. Here we shall only note that the youngest 

 and smallest joints always lie next the head, 2 and FlG - 225. Tcema 



^ ' serrata twenty hours 



only become mature after they have been pushed out old, with incipent seg- 

 to some distance from their point of origin. 3 On the mentation - ( * i<>.) 

 other hand, the smallest, even microscopic, joints show essentially 

 the subsequent musculature, though individual bands may be un- 

 equally developed. The main mass of parenchyma consists, of course, 

 of distinctly nucleated cells, which multiply rapidly, and pass subse- 

 quently into the connective tissue ground-substance, or into the 

 various generative organs. For the latter purpose, the cells group 

 themselves in strands or masses, and either directly form the repro- 

 ductive material, or, through the formation of an inner cavity, become 

 epithelial (endothelial) structures, which occasionally acquire a more 

 or less firm cuticular layer. 



We have already seen some instances of the way in which the 

 form and independence of the joints may vary, and the same manifold 



1 I speak purposely of the " segmentation of the neck," for the widely prevalent 

 opinion that the joints arise through budding is strictly speaking incorrect. The 

 joints are not produced by buds, that is to say, not by portions which are appended to the 

 parts already present, and which develop without their participation ; they arise by the 

 growth and modification of parts already present (the so-called "neck "), as has been 

 shown above (p. 380) in the case of Tetrarhynchus, and as may be similarly, though less 

 conspicuously, demonstrated in the case of Tcenia. 



2 Here I ought perhaps to refer to Moniez's opinion, according to which (Bull. sci. 

 dep. du Nord, p. 293, 1879) the head of the tape-worm represents not the anterior, but 

 rather the posterior end of the body. Moniez regards the tape-worm as a simple animal, 

 and denies the existence of an alternation of generations, and is able by means of the above 

 theory to remove the difference between the tape-worms and the segmented worms, as 

 regards the position of the buds, which, as is well known, are in the latter always before 

 the anal segment. 



8 [The first formed joints are very commonly, though to a different extent in diverse 

 species, sterile, and smaller than the later ones. Where they are numerous, as, e.g., in 

 Tcenia perfoliata, the appearance of the young colony is so different from that of the 

 older, that they have been regarded as different species. Thus, Tcenia plicata, Auct., is 

 only the young stage of T. perfoliata with a large number of sterile joints. R. L.J 



