90 ` The American Naturalist. [February, 
loose joints, where the hard parts scarcely come in contact with 
one another, and cases of degeneration of the hard parts, lead- 
ing to total disappearance of a previously existing joint. 
In the annelides, from which, there is no doubt, the arthro- 
pod branch sprang, we find no deposit of inorganic salts in 
the epidermis. (Ido not regard the hard, calcareous, and 
other secretions of tubicolous annelides as entering into this 
question, as they are merely protective coverings, not acted 
upon by the museles of the body, nor is there even any mus- 
cular connection with it.) The outer layer of the body is 
generally of a horn-like character, adhering closely to the — 
secretive cells of the epidermis, very flexible and thrown into — 
folds by the vermicular motion of its possessor. In the — 
leeches, the body consists of a flexible cylinder, made up of 
two sets of muscles, an outer longitudinal cylinder and an — 
inner cylinder of circular fibers, the contraction of which, — 
causes the animal to increase in length, while shortening is. 
effected by the contraction of the longitudinal layer. The — 
external surface of the medicinal leech, for example (see fig. 1) a 
- is thrown into a regular series 0 
very fine folds, extending across the 
longitudinal axis of the body 
These folds do not correspond in- 
OE are a S 
to one somite. When the ne 
shortens its length, these folds 
closely eh eg when extension 
takes place, the folds are flatten 
spread open, although not wh lly 
disappearing, as they are a fix 
quantity so to speak. I be 
