506 MR. F. E. BEDDARD ON THE 
Postscript—(added Nov. 21). 
(1) Since writing the above account of the anatomy of Plewrocheta, I find 
that a memoir has been overlooked which explains the anomalous structure of 
the dorsal vessel. This memoir is by Dr. F. Vespovsky, and is abstracted in the 
Journal of the Royal Microscopical Society for 1880. Dr. VEspovsxy, in study- 
ing the development of Criodrilus, discovered that the dorsal vessel is formed 
by the coalescence of two completely separate rudiments—the same mode of 
development had been previously shown by Kova.Levsky to take place in 
Lumbricus—these facts, VEsDOvsKY points out, are of extreme importance in 
considering the relationship between the Annelida on the one hand, and the 
Vertebrata and Crustacea on the other, since in the Vertebrata generally, and in 
Apus among the Crustacea, the heart is formed in the same way by the 
coalescence of two rudiments which at first are distinct. The Hermellida, 
according to DE QUATREFAGES, possess two dorsal vessels in the posterior part 
of the body which are joined into a single tube anteriorly, and in Pleurocheta 
the same embryonic character is shown in the dorsal vessel, but in a much 
more marked degree ; it is evidently formed of two incompletely fused tubes 
(cf. Pl. OXY. ties). 
(2) Ihave attempted to show that there is a fundamental similarity in the 
structure of the muscles of the longitudinal coat throughout the Oligocheta, and 
that Lumbricus is not to be regarded as differing essentially from other 
Oligocheta in this respect. This conclusion is quite justified by the facts 
brought forward by the Drs. Hertwie in their “ Coelomtheorie,” * which I had 
not read at the time that I was preparing this paper. These authors show the 
general similarity, both in structure and development, that exists between the 
muscles of the Annelida, Chetognatha, Vertebrata, and other orders in which 
there is a true enteroccele. In these groups the muscular tissue is developed from 
the epithelial lining of the perivisceral cavity, and almost without exception 
from the parietal layer; the elements—the fibrillee—are invariably bound together 
to form higher unities ; such as, for example, the fibril bundles of many verte- 
brates, and are also characterised by the regularity of their arrangement. In 
the Mollusca and Platythelminthes, and other groups in which there is not a 
true enteroccele, the muscles are developed from cells of the ‘‘mesenchym,” 
which have the character of connective tissue cells ; they frequentlypossess 
longitudinal striz, which are not, however, to be regarded*as expressing a 
fibrillation, and are always arranged irregularly, crossing each other in various 
directions, and contrasting very strongly with the regular arrangement of the 
“ epithelial” muscles of the Annelida, Vertebrata, &c. 
* Jen, Zeitsch. fiir Natirwiss., 1881. 
