15 



generally deltoid, tapers posteriorly, and is broader than long. It is 

 also frequently deeply emarginated in front. Whenever we find 

 several of these characteristics together we may be reasonably sure that 

 the species belongs to the genus Mesenchytrceus. 



Spermathecce . These organs show a great variation in form and in 

 the number of diverticles. The latter offer a most convenient character 

 upon which to base a systematic arrangement of the species. In the 

 following I have adopted the number of diverticles of the spermatheca 

 as a most convenient characteristic for the different groups. There are 

 also points in the structure of the spermatheca which are of great 

 interest. In a large block of species, which also otherwise seem to 

 be related, the terminal ampulla of the spermatheca is greatly enlarged 

 and extends backward through a number of somites. As might be 

 expected, nearly all such spermathecae are closed and do not connect 

 with the intestine. The exception is found in M. vegce in which the 

 spermatheca is connected with the intestine by a narrow duct, which, 

 however, springs out laterally from the ampulla instead of from its inner 

 apex. There is some little reason to suspect that this enlargement of 

 the spermathecae in this genus may have been overlooked in some 

 species, and that some spermatheca} which have been described as 

 short and as immediately connecting with the intestine, in reality are 

 greatly prolonged posteriorly. The part adjoining the diverticles is 

 always narrow and closely approaches the intestine. This peculiarity 

 causes it to tear readily and I am satisfied that some such torn sperma- 

 thecae have been considered as entire. A similar enlargement of the 

 spermathecae is not known to exist in any of the other genera of this 

 family. 



Spermtducal apparatus. The spermiducal apparatus in Afesen* 

 chytrceus is as a rule most characteristic. This refers especially to 

 the sperm-duct and to the various glands connected with it. In nearly 

 all species of this genus there exists an enlargement of the sperm-duct 

 just before it enters the penial bulb. I have retained for this enlarge- 

 ment the name ' atrium.' In this atrium there open in many species 

 glands, in form, size, and structure resembling the atrial glands of 

 Limnodrilus . In some species there are only a few glands, in others 

 there are as many as fifteen or more. The atrial glands consist of fas- 

 cicles of unicellular glands, each cell opening independently of the 

 adjoining cells. The glands open in various places. As a rule they 

 penetrate the atrial wall in a fascicle surrounded by circular muscles, 

 though these latter may be absent. After having penetrated the atrial 

 wall, the ducts of the glands may open into pockets between the cpi- 



