32 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the segmentation cavity and represents the primary coelom ; the peri- 
cardial cavity arises independently of the peribranchial cavity, but is 
probably analogous with it. 
The bud developes from two layers, ectodermic and endomesodermic. 
The mesoderm is separated by the formation of the lateral diverticula. 
From the ectoderm, the body-wall, the inhalent and the exhalent tubes, 
and the brain develope. From the endoderm, the gut, the branchial sac, 
the hypophysis, &c., develope. The mesoderm forms the peribranchial 
sac and the heart. In the wall of the peribranchial sac the reproductive 
cells occur, and give rise on the one hand to the ova, on the other hand 
to the endo-mesodermic portion of the bud. The thin ectodermic layer 
which shares in the bud secondarily acquires an embryonic character. 
/ 3 . Bryozoa. 
Structure of Rhabdopleura.* — Dr. G. Herbert Fowler has a pre- 
liminary note on his observations on Rhabdopleura ; all the new 
anatomical features which he has been able to detect are in entire 
agreement with the structure of Cephalodiscus , and Rhabdopleura may 
be taken to form a third member of the order Hemichordata. 
The epistome is found to correspond to the proboscis of Balanoglossus 
and Cephalodiscus, and, as in them, it contains a portion of the coelom, 
completely shut off from the other portions. The collar region contains 
the central section of the coelom, divided into right and left halves by 
median septa ; each half communicates with the exterior by means of 
its own collar canals. On the posterior face of this cavity there is an 
ectodermal thickening, which corresponds in position with the nerve- 
plate of Cephalodiscus and the nerve-tube of Balanoglossus. There is 
a rod-like structure, apparently half cellular, half gelatinoid, which 
corresponds in origin, structure, and jmsition with the notochord of 
Cephalodiscus. The most notable part of the intestine is a short semi- 
circular diverticulum, which occurs also in the just-named form. The 
absence of a proboscis pore or pores, and the absence of gill-slits are 
two negative characters in which Rhabdopleura differs from the two 
known Hemichordata, but Dr. Fowler thinks the points of agreement 
are so striking that it is impossible to separate the three organisms. 
Arthropoda. 
Origin of Tracheae of Arthropoda from Setiparous Sacs.j — Mr. H. 
M. Bernard attempts to solve the following problem : — “ Certain 
Chaetopod Annelids migrated on to the land at an early geological 
period, and are now represented by the Tracheata, so called because of 
their breathing organs which are chitin-lined invaginations of the outer 
cuticle — whence came their respiratory organs ? ” 
He deduces the tracheae from setiparous glands, and urges that this 
harmonizes many of the anomalies presented by the tracheal systems of 
the Arthropoda. The diffuse arrangement of the tracheae of Peripatus 
is derived from the bristle-glands which were scattered over the surface 
of the body, while the regular metameric arrangement of the tracheae 
* Proc. Roy. Soc. Loud., lii. (1892) pp. 132-4 (3 figs.), 
f Zool. Jahrb. (Anat. u. Ontog.), v. (1892) pp. 511-24 (3 figs.). 
