I116 General Notes. [ November, 
directly or indirectly from the archenteron. It is also assumed 
with Sedgwick! that the most primitive form in which an imper- 
fect approach towards the differentiation of a body-cavity is evi- 
dent, as paired pouches of the archenteron, still opening into the 
latter, is seenin the bilaterally differentiated Actinozoa. Itis 
further assumed that this modification of the archigastrula, as the 
primitive gastrula may be called as defined by Haeckel, is the first 
intimation which we have in any existing type, permanently repre- 
sented in the ascending scale of morphological differentiation of 
organisms, of the permanent assumption of bilaterality. It is also 
assumed with Sedgwick that the mouth of such a form was elon- 
gated in an antero-posterior direction, thus leading to the differen- 
tiation of a permanent mouth and anusat the opposite ends of the 
original slit-like mouth of such a form. The circumoral band 
of sensitive tissue is also assumed to have given rise to the 
median nervous system of Ghordata and Achordata. In the — 
former median concrescence of the originally paired cords has 
been complete, and in the latter incomplete, so as to give rise to 
circumoral and circumanal nerve rings and a pair of ventral 
ganglionated cords. It is thus made obvious that I assume in a 
general way that the hypotheses propounded by Sedgwick aresup- 
ported by a very large body of evidence and enable us to interpret 
and reconcile with great readiness the conclusions of biologists 
in reference to the development of other structures, especially the 
excretory, generative and appendicular organs. e evolution of 
the first two, the trachea of insects, the branchiz of various forms, 
the origin of Metameric Segmentation and other morphological questions. 
from the Morph. Laboratory in the University of Cambridge, 11, 1884, pt. & 
x and xi. Also in Quart. Jour. Mic. Science, 1884. 
ic. Science, I 
