356 MOORE. [Vol. XIII. 



B. illuminatus is the most actively contractile, and may be seen 

 in specimens slightly flattened under a cover glass to contract 

 spasmodically with some regularity, and as frequently as two 

 or three times per minute. The diastole takes place slowly, 

 the systole suddenly and vigorously. Figs. 48 and 49 represent 

 the appearance of the vesicle in diastole and systole respec- 

 tively, as seen in dorsal views. In no case has any contraction 

 of the efferent duct been noticed. 



In both B. philadelphicus and B. illuminatus three or four 

 small, unicellular, pear-shaped glands {g) are related to the 

 anterior vesicle. In the former they open into the lower end 

 of the vesicle at the point where the efferent ducts enter, in 

 the latter either into the neck or just before the pore. No 

 similar cells have been found elsewhere in relation to the 

 nephridia. 



8. Investments of the Nephridia. 



There are two investing membranes, an internal connective- 

 tissue sheath, which covers all parts, and a peritoneal endothe- 

 lium, which, of course, is confined to the coelomic portions. 



The former is similar in all species; it consists of a thin 

 granular layer, becoming at places fibrous, and here and there 

 incomplete for small spaces, as for example, at the ends of the 

 tubule loops. In the species with compacted plexus masses, 

 this sheath is rather thicker, more richly nucleated, and more 

 uniform. Any irregularities of surface, clefts and spaces 

 between the tubules are filled by connective tissue (Figs, i, 

 ctn, ctl, 13-15), and in B. illuminatus sheets extend into the 

 loose, open plexus mass; even in Branchiobdella scattered 

 nuclei will be noted here and there within the mass. The 

 tubule loops are everywhere uniformly ensheathed (Figs. 20, 

 24, 27, ct), but it is interesting to note that this layer is absent 

 from the expanded portion of the funnel, beginning only at the 

 point where the latter meets the efferent duct (Fig. 10). A 

 thin lamina also lies between the double peritoneal sheets 

 which suspend the plexus mass from the body-walls. The 

 intermuscular tubule is likewise invested in a connective-tissue 

 coat continuous with that covering the coelomic structures 



