STRUCTURE OE THREE NEW SPECIES OP EARTHWORMS. 123 
ture between the glandular cells which compose the greater 
part of the atrial epithelium in Eudrilus and the cells which 
cover the atrium in the Lumbriculidae and in Mouili- 
gaster (PI. XII, fig. 11). Outside this muscular layer, which 
covers the glandular lining, are faint traces of a peritoneal 
investment, and it is this which is the homologue of the 
glandular sheath in Rhynchelmis, Moni ligaster, &c. 
The explanation of the difference in the structure of the atrial 
epithelium is, as it appears to me, quite another one. 
It has been conclusively proved by Yejdovsky that the atrium 
in the Tubificidse is formed by an ectodermic involution just 
as are the spermathecae and the “vesicle” of the nephridia, 
and as a consequence it retains the structure of the integu- 
mental layers. The ciliation of the lining epithelium is par- 
ticularly interesting in this connection, because it often hap- 
pens that ectodermic involutions, owing to the protection which 
they afford, retain the ciliated condition which is lost on the 
general body surface. Moreover, the cells in the distal glan- 
dular part of the atrium in the adult have more completely 
retained the characters of the epidermis than in the proximal 
region, where it has undergone secondary modifications in 
connection with the formation of the penis. 
In Eudrilus the male reproductive pores are intraclitelline. 
It is a fair assumption to suppose that the atrium is invaginated 
from the ectoderm, and it will therefore retain to a certain 
extent the structure of the body wall as it does in Tubifex. 
The ectodermic cells in the young embryo, at the point where 
the atrium is invaginated, have the potential capacity of de- 
veloping into the complicated clitellar epidermis ; it is there- 
fore not surprising to find that the invaginated cells also retain 
this capacity, and ultimately form an epithelium nearly identical 
in structure with the clitellum. The objection that those cells 
which are nearest to the point of invagination are most unlike 
the clitellar epidermis is to be met by reference to Tubifex. 
The absence of cilia may be reasonably accounted for on the 
supposition that Eudrilus is farther removed from the 
ancestral ciliated condition than Tubifex. 
