BLASTOPORE, MESODERM AND METAMERIC SEGMENTATION. 25 
in formative masses, which afterwards grow to line the body 
cavity (in Echiurus, however, the mesoderm much more nearly 
keeps pace with the ectoderm than in Amphioxus). Thus, I 
consider that in Chsetopoda, Arthropoda, and Yertebrata the 
mesoderm will tend to be left in regular pairs of masses, while 
the elongation of the body is taking place ; I seek to explain 
the whole of the facts of metameric segmentation as arising 
from the necessities of development. 
In Phoronis the external openings of the nephridia are parts 
of the blastopore. The same considerations w'hich have been 
applied to the mouth and anus, mesoderm and metameric 
segmentation bear on the question of the origin of nephridia. 
The nephridial portions of the mesoderm may remain in 
various positions, in other words, the nephridia may in accord- 
ance with my hypothesis arise as single or as serial ectoblastic 
or endoblastic pairs of pouches with or without connecting 
longitudinal canals or cords. 
Instances of most of these possible modes of origin have 
been described in the different groups of animals. The bearing 
of the above facts and hypotheses on the nervous system, gill- 
slits, notochord, and other organs, will be obvious to anyone 
who has followed me so far. I hope to be excused from 
entering into completer discussion of my hypothesis by reason 
of the want of books of reference in my present situation out 
of the world. 
Summary. 
Facts in the development of Phoronis — 
1. The blastopore gives rise to both mouth and anus. 
2. The mesoderm arises in an anterior pair of endoblastic 
modified diverticula, and in a posterior pair of ectoblastic 
diverticula connected by a few mesodermic cells derived from 
the middle of a primitive streak. 
3. The nephridial openings to the exterior are parts of the 
blastopore. 
Preliminary interpretation suggested by these facts of the 
development of Phoronis — 
