HETEROMORPHOSIS 127 
grow almost perpendicularly away from their substratum. 
If, in addition, the separate stems stand very close together, 
as is usually the case, the contact of the polyps with each 
other influences their orientation. This has the same effect 
as would be brought about by causing each separate polyp 
to grow in a narrow hollow cylinder. The individual stems 
must thus not only grow away from the surfaces to which 
they are attached, but they must grow away from it in 
approximately straight lines. 
3. Dalyell has observed in Tubularia indivisa—a form 
very similar to Tubularia mesembryanthemum—that the 
polyps drop off after they have existed a certain length of 
time, and that after a longer or shorter period new polyps 
are formed in their places. As soon as a new polyp has 
been formed, the stem begins to grow in length immediately 
under it. The growth continues as long as the polyp exists; 
as soon as it drops off, growth ceases.’ 
I observed the same condition of growth in Tubularia 
mesembryanthemum. The longitudinal growth of the stem 
was continued to a region just beneath the polyp, and it 
continued as long as the polyp existed; when the latter 
dropped off, growth ceased; when a new polyp was formed, 
the stem again grew in length. In the bioral polyps an 
increase in length occurred simultaneously at both ends of 
the stem, so that these stems reached a much greater length 
in a shorter time than any of the normal specimens that 
were ever brought to me by the collectors of the Zodlogical 
Station in Naples. That the stem grows in length close 
behind the polyps at both ends of the bioral animal is 
clearly shown by the fact that the newly formed part is thin 
and transparent, and thus can be readily distinguished from 
the older opaque portions of the stem. Therefore in its 
growth also the aboral pole of Tubularia behaves like the oral. 
1 Rare and Remarkable Animals of Scotland (London, 1847). 
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