130 PROCEEDINGS OF THE ACADEMY OF [1899. 



stricted at the septa and is occupied, in addition to the ova, by one 

 of the sperm ducts. In those specimens in which it is best devel- 

 oped it ends in somite XXII, but in some specimens it is less devel- 

 oped, and doubtless varies as in other species of Mesenchytrmis. 

 Sometimes ova are present in the posterior part only, in which case 

 the anterior part, occupied by the sperm duct, is difficult to dis- 

 tinguish. 



The paired spermathecse (fig. 5) open externally between IV 

 and V, and on a level midway between the dorsal and ventral 

 setpe bundles. The epidermis about the opening is thickened in 

 an elliptical area, owing to the presence of numerous uniceSlular 

 glands. The stalk has a narrow lumen and thick walls made up 

 as follows: (1) A thick lining cuticle; (2) a very deep epithe- 

 lium consisting of a layer of cells divided into an inner (lumenal), 

 which is unstained, and an outer (basal), stained and nucleated 

 zone; at the plane of separation is a ring of pigment granules; (3) 

 a very regular layer of longitudinal muscle fibres, one deep; (4) 

 a layer of enlarged peritoneal cells, largest posteriorly and dor- 

 sally, and much smaller on the side toward the septum. The stalk 

 passes mesiad along the septum, then turns dorsad and caudad, the 

 muscular and peritoneal glandular layers cease, and the Avails be- 

 come thinner as the organ expands into the ampulla, which is some- 

 times very spacious, sometimes, as in the specimen figured, quite 

 contracted. At the point where the stalk expands into the ampulla 

 arise three slender, thin-walled diverticula, usually two from the 

 outer and one from the inner (mesial) face. They vary in length, 

 the longest about equalling the stalk. The ampullaj of the two 

 sides open into one another, and the common sac thus formed by a 

 small pore into the oesophagus on its dorsal side,^ The epithelium 



^ I find upon reexamining the material that several of the more slender 

 and lighter-colored worms which I had taken for immature individuals pos- 

 sess spermathecaj of quite a difi'erent form. They are much smaller than 

 those of 3/. solifugus and lack the diverticula entirely; they are simple 

 club-shaped sac?, without spedally enlarged ampulla;, and communicate 

 neither with one another nor with the oesophagus. The epidermis about the 

 spermathecai openings is not thickened. The male genital organs also 

 differ ; the saccus ejaculatorias is smaller, the walls of the atria less thick, 

 and the external pore much less conspicuous. The posterior border of the 

 supra -oesophageal ganglion is concave. These characters are exhibited 

 by three specimens. The poor state of preservation of the specimens per- 

 mits no further description. For this species the name Mesenchytroeus 

 niviis, which has not yet been published for the other, is proposed. 



