286 Proceedings of Royal Society of Edinburgh. [sess. 
a simple nervous system which at least in some ( Argiope , Shipley)* 
is in continuity with the ectoderm. It consists, typically, of a 
protomeric ganglion (supra-oesophageal) a post-oral mesomeric ring 
with one or two mesomeric ganglia (Yan Bemmelen,f Jouhin), from 
which the posterior part of the body is supplied with simple nerves. 
The arms of the lophophore, consisting of epistome (protomere) and 
mesomere, are supplied by nerves from both the supra- and the sub- 
oesophageal ganglia (Yan Bemmelen). 
The vascular system, if existent, is in the rudimentary condition 
(Bloclimann,| Hancock §) of simple sinuses between the coelomic 
walls, or splits in the mesenteries. In several forms there has 
been described, between the protocoele and the mesocoele, a compli- 
cated peri-oesophageal sinus or plexus of sinuses, which is in direct 
connection with, and forms the posterior part of, the protocoele, with 
which the blood system comes into connection. More definite 
knowledge is required upon this point, hut it is possible that further 
research may reveal a trace of the archimeric heart or suhneural 
sinus in this position. (Of., specially Shipley, loc. cit., pi. 39, 
fig. 1, b.v., and Beyer, Stud. Biol. Lab., J. Hopk Lab., 1886.) In 
the adult, at least, there appears to he no trace of a suhneural 
gland. 
The gonads are confined to the metacoeles and the metacoelic 
nephridia act as gonaducts. 
Ontogenetic. 
Tree larval forms are found in the Brachiopoda, and all those 
as yet known present a well-defined division into three segments. 
This has been especially emphasised by Morse || in his attempt to 
prove the annelid affinities of the group (fig. 12). This comparison 
of Brachiopoda with larval Annelida acquires a new significance 
in the light of the theory here propounded, as will he shown later. 
Thecidium (Lacaze Duthiers) (fig. 11), Terebratulina (Morse), (fig. 
12), Argiope (Kowalewski, Shipley) (figs. 9, 10), present larval 
* Mittheil, Neap. Zool. Stat. , I Y. 
t Jenaische Zeitsch., 1883. 
+ Zoolog. Anzeiger, viii., 1885. 
§ Phil. Trans., 148, 1858. 
|| Pros. Boston Soc. N.H., xv. p. 315-372. 
