704 Laura FLORENCE 
basal plate and parameres, and the slender part has passed along the center 
of the vesica, where it is surrounded by a sheath composed of slender 
muscle fibers. This sheath originates as two lateral bundles on the 
proximal border of the basal plate and is: inserted as fine strands on the 
wall of the vesica at its junction with the penis. 
At the close of copulation the protractor muscles and the body muscles 
relax, and the coelomic fluid passes back into the body. The vesica penis 
is drawn to its resting position by the contraction of the muscle sheath of 
the slender part of the ejaculatory duct, as well as by the contraction 
of many fine muscle fibers which are inserted on the surface of its ante- 
rior half and have their origin in the dorsal anterior border of the basal 
plate. When at rest these muscle fibers form a thick layer on the anterior 
region of the basal plate and a thin layer between the vesica penis: and 
the basal plate. Some muscle fibers originate in the ventral body wall 
between segments 6 and 7 and are inserted in the anterior border of the 
basal plate, and these by their contraction bring the framework of the 
apparatus to its resting position. ; 
The histological structure of the mesodermal organs shows some interest- 
ing features. The testes are surrounded by a three-layered wall — an 
inner slender epithelium, a very fine basement membrane, and a peritoneal 
wall in which there is no pigment. Fat bodies are closely apposed to the 
dorsal surface of the testes, and among them, as also in the peritoneal 
wall, tracheoles are very numerous. The contents of the testes consist 
of cells and developed spermatozoa, which for the most part lie in clusters 
of from six to twelve individuals. This is similar to the finding of Landois 
(1865 a:53) in Pediculus, and is common in insects. Each spermatozoon 
has a rod-shaped nucleus in the head, which takes the hematoxylin stain 
so intensely as to appear black. Anterior to the nucleus can be distin- 
guished a small area of cytoplasm staining a bright pink with eosin just 
as the tail stains. No middle piece can be distinguished. These clusters 
of spermatozoa are in that half of each testis which lies next to the vas 
deferens, and appear to rest in a matrix of nutritive cells with very pale- 
staining nuclei. The remainder of each testis is filled with cells typical 
of the different zones of development. At the base there is a very small 
cluster of spermatogonia, followed by spermatocytes of both orders in 
process of division and reduction, and then a small section of spermatids. 
