1895-96.] Mr Masterman on the Structure of Aciinotrocha. 129 
On the Structure of Adinotvocha considered in relation 
to the suggested Chordate Affinities of Phoronis. By 
A. T. Masterman, B.A., Lecturer and Assistant Professor 
of Natural History in the University of St Andrews. 
(Read June 15, 1896.) 
Some little time ago a preliminary note upon Phoronis was read 
before the Royal Society, and therein it was stated that a study of 
the anatomy of this group had led one to believe that its nearest 
phyletic allies were to be found in the Hemicliorda , a group com- 
prising a few species with primitive chordate characters. 
In attempting to class Phoronis with these, an important diffi- 
culty presented itself. It has been usual to define the Chordata as 
having three essential characters in common, either temporarily in 
ontogeny or permanently throughout life, i.e., a dorsal nervous 
system, paired gill-slits, and a notochord. 
With regard to these features, there can be no doubt that the 
nervous system of Phoronis is dorsal in position, and its arrange- 
ment is exactly comparable to that of Balanoglossus ; but, on the 
other hand, there are no gill-slits in the adult Phoronis , and the 
notochord is as certainly absent. 
In the case of the gill-slits, we find upon critical examination 
that they are extremely variable in extent of number and position 
in the different Chordata, and in one species, Rhahdopleura, they 
are, according to present knowledge, entirely absent. For these 
reasons it is well not to insist on the presence of gill-slits as an 
essential character. With respect to the notochord a different con- 
dition prevails — the liypoblastic origin and vacuolated character of 
this organ mark it out from all other skeletal structures, and it 
figures more or less prominently in all the Chordata. This fact, 
and the consideration that Phoronis has a degenerate sedentary 
habit, with a well-developed mesoblastic skeleton of chondroid tissue 
comparable to that of the Yertebrata, replacing, in them, the 
notochord, taken in conjunction with the remarkable anatomical 
resemblances to Balanoglossus and Cephalodiscus , seemed to justify 
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