NOTES ON THE HISTllIOBDELLID^. 
205 
described below in the account of the cervical glands, behaves 
like a closed cavity. In the head the ccnlom is represented 
by an extensive cavity on the ventral side, with lateral 
extensions round the jaws and their muscles, opening dorsally 
into a considerable median cavity situated beneath the brain. 
These head-cavities are quite clear except for some muscular 
fibres. They have a splanchnic layer of coolenchyme like 
that covering the intestine, forming a capsule enclosing the 
jaws and their muscles, and the existence of a parietal layer 
is indicated by the presence of very sparsely distributed 
nuclei. 
Briefly stated, the arrangement of the coelenchyme and its 
relations to the coelom in Stratiodrilus are as follows: 
The coelom, which is probably a schizocoele, is not lined in 
any part by an epithelium ; but the coelenchyme, a nucleated 
substance of undifferentiated, finely fibrillated material, with 
no trace of division into cells, partly takes the place of such 
a membrane. It forms a thin splanchnic layer investing the 
whole of the digestive canal and the ovary and testis. Its 
somatic portion, which assumes the character of a continuous 
layer on the ventral side only, is intimately connected with 
the longitudinal muscular fibres of the body-wall, of which it 
constitutes the formative (myoblastic) material. 
As I pointed out in my previous paper, this condition, in 
which the same elements play the part of myocytes and of 
somatic coelomic epithelium, is essentially not dissimilar to 
the condition in the larva of Polygordius (Praipont, 4), in 
which a single layer of cells gives origin both to the longi- 
tudinal muscular fibres and the somatic layer of the coelomic 
epithelium. To judge from certain of Eisig’s figures (2), the 
same holds good of the Capitellidae. 
Pierantoni (8) describes a complete peritoneal layer in 
Protodrilus, but does not enter into an account of its 
relationships to the muscular layers. In his text-fig. 1, p. 33, 
he shows a cell with a flattened nucleus lying within and 
distinct from the protoplasmic parts of the longitudinal 
muscular fibres, and refers to it as representing the peri- 
