ANATOMY AND HISTOLOGY OF PLEUROCHATA MOSELEYI. 507 
On plate iii, of the “Ccelomtheorie” there are figures of the muscles in 
a great variety of animals belonging to different groups, and a comparison of 
fig. 13, with my drawing of the muscular compartments of the young Pleuro- 
cheta (Plate X XVI. fig. 15), shows a very striking resemblance, and it seems 
highly probable that the muscular compartments of Pleurocheta are homologous 
with the “ Muskelkistchen” of Petromyzon. On page 6 of the ‘ Coelomtheorie,” 
the authors sum up briefly the account of the anatomy and development of the 
muscles given in a previous memoir on the Chetognatha:—“ As in the Actinize 
from the epithelial cells of the diverticula of the Archenteron, so in the Cheetog- 
natha from the parietal epithelial layer of the Coelome (=Somatopleure) are 
secreted muscular fibrillee, which become united into a lamella. In the further 
process of development of this lamella it becomes folded, and gives rise to 
muscle plates (Muskelblitter.)” This statement surely is not reconcilable 
with that made on page 63 of the same memoir :—“ Each muscle plate (in 
Petromyzon) is formed by the neighbouring borders of two myoblasts .... . 
the close resemblance to the muscle plates in the Chetognatha, many 
Nematoda, and the Annelida, is so obvious, that it is sufficient merely to have 
called attention to it.”* It is certainly quite true that the resemblance here 
remarked upon is very close, but of course there can be no real similarity in detail 
if the development in Sagitta is such as it is stated to be in the former of the two 
passages cited; there is clearly no “folding” in the case of Petromyzon. Still 
less can there be any comparison made between Lumbricus and Petromyzon, since, 
according to the Drs. HErtwie, a second folding has taken place in Lumbricus, 
so that a “ fibril” here is not the-equivalent of a fibril in Sagitta or Petromyzon ; 
at the same time, the letter fis made use of to denote the fibrils in all the 
three types, which is rather confusing, and might lead one at first to believe that 
they were considered to be homologous structures. The development of the 
muscles in Lumbricus is at present not known, so that any comparisons made 
with other forms can only have a slight value; the evidence that we have, 
however, appears to me to point to the conclusion that there is no need to 
imagine a second folding in the longitudinal muscles of Lumbricus. Had there 
been a second folding, we might have expected to find a septum of connective 
tissue between the secondary lamella, continuous with that separating the 
primary lamellz ; but this does not seem to be the case ; although CLAPAREDE 
describes the central fibrous septum as sending branches between the fibrils, 
the inter-fibrillar substance is of a very different appearance from that forming 
the septum, according to the figures given on plate iii. of the “ Ccelomtheorie.” 
It is a delicate granular substance with frequent nuclei, and is more like 
the part left over in the original myoblast after the secretion of the muscle 
fibrils. At any rate, the absence of capillaries and pigment granules, which are 
* Jen. Zeitsch. fir Natiirwiss., 1880, 
