64 MORGAN. [Vou. IX. 
a landmark by which we are able to determine the subsequent 
growth of this region. 
This brings us to the third period of growth, where the body 
again begins to enlarge. The enlargement here is brought 
about in a different way. The tucrease in size takes place in 
all the organs at the same time, and not as in the early stage, 
at the expense of one set of structures alone. 
The animal gvows by the increase in size of all the organs 
of the body. During this period of growth new organs again 
put in their appearance, noticeably the collar-pores, gill-slits, 
notochord, and nervous system. There is one fact that has 
been clearly seen in the later period of growth of the young 
Balanoglossus that I regard as of capital importance: The 
increase in length of the young worm ts due to a general inter- 
stitial growth, the elongation of the posterior metamere of the 
young worm ts not due to apical growth. We are able to make 
this statement positively, and I think it must be accepted with- 
out question. The position of the circular ciliated band can 
be followed, as already described, into the later stages, and by 
this means (taken in connection with the general phenomena 
of growth) we may make the definite statement given above. 
An interesting comparison might be made, I think, as to the 
method of elongation of the body in such groups as the Verte- 
brates, Annelids, Nemerteans, and Holothurians. The data 
pointing to an apical elongation in the Vertebrates and Annelids 
rests on a sufficiently firm foundation. How much, in addition 
to this, an interstitial increase helps in the elongation, has not 
been studied, I think, with sufficient care. In the Nemertians 
and Holothurians, I do not know of any data sufficient to 
warrant a definite statement. 
In an earlier paper, I have expressed my opinion that the 
elongation of the body of Balanoglossus posteriorly is a 
secondary growth, and has appeared as a more recent adapta- 
tion to a life in the sand; and it is connected, perhaps, with 
the fact that large quantities of sand are taken into the diges- 
tive tube with a relatively small return of digestible material. 
The immense elongation of the last pair of body-cavities is 
merely then due to the vegetative elongation of the posterior 
metamere of the body. 
