494 MR. A. D. MICHAEL ON THE 



plainly i-inged : this feature is frequently found in tlie principal ducts of the Acarina, in 

 which order a ringed tube is more likely to be a duct thau a trachea. 



The course of the main common duct is as follows : starting from the tubular gland 

 it runs forward a little, and then curves round the outer side of the acetabulum of the 

 first leg ; it then runs forward along the shoulder of the creature, usually in a some- 

 what undulating line, but on the whole tending slightly inward, until it enters the 

 rostrum and apj)roaches very near to the palpus ; it then turns suddenly inward toward 

 the median line, and runs across the upj)er surface of the chitinoiis roof upon which 

 the mandibles rest, passing beneath the proximal ends of the mandibles ; on reaching 

 nearly to the median line the duct turns slightly forward, joins its fellow from the other 

 side of the body, and the two together discharge into a membranous space upon the 

 chitinous roof and between the mandibles near their proximal ends. 



The Henifonn Gland. — A short distance anterior to the acetabulum of the first leg 

 the common duct receives the special duct (fig. 9, dr) from a very large gland (tigs. 8, sgr); 

 this duct is small and fine, does not show any ringed structure, and enters the upper 

 surface of the common duct. The ghrnd which discharges by this small duct is one of 

 the largest in the creature ; it is clearly the homologue of the " reniform salivary gland " 

 in Thyas ■petropMlus, and of Schaub's " larger dorsal mouth-gland " in SyJrodroma. 

 Pagenstecher, in TromJddium, and Guddon*, in Tyroglyphns, have also figured homologous 

 glands, but without specially naming them. I liave retained the name " reniform gland," 

 but the gland in Bdella (fig. 9) is somewhat different in form from any of those hitherto 

 described : it cannot be called reniform ; it varies somewhat in shape, but is usually irre- 

 gular, approaching an oval, and is flattened, its thickness not averaging above a third of 

 its breadth ; it is placed on edge in the body, which is not the case with any of the other 

 recorded homologous glands, and has the more pointed end forward ; its inner surface 

 is nearly flat, but the outer surface is more convex ; probably adapting itself some- 

 what to the form of the side of the body. This gland stains only slightly with 

 hsematoxylin or carmine : it is composed of extremely large, almost gigantic, cells 

 radiating from a centre ; these cells have an extreme length varying from about "15 mm. 

 to over '2 mm., and an exti-eme width of about half those measurements, and have 

 large, clear, elliptical nuclei of about "03 mm., somewhat regularly arranged, with very 

 distinct nucleoli of about "015 mm. This gland always contains a chamber more or less 

 wedge-shaped {cv) on its outer edge ; this chamber is apparently one of the cells broken 

 down and transformed, for it often contains a nucleus, but it is usually filled with dark- 

 coloured and highly-refractive granules, measuring from i ^ to 7 /i ; it communicates with 

 a small central pocket, from which the duct starts. Sometimes this wedge-shajied 

 chamber appears to invade and absorb the adjoining cells, and then becomes very large. 



The anterior Salivary Gland. — Just before entering the rostrum the upper surface of 

 the common duct receives another special duct (fig. 8), which is much shorter and still 

 finer than that from the " reniform gland " : the gland {sga) from which this duct leads 

 will be the homologue of the " quadrate salivary gland '' in Thyas petrophilus, and of 



* " Beitrag zur Lehre von dcr Scabies," Wiirzburgcr medicinische Zeitsch. 1861, p. 801 ; and "Zweite vermehrte 

 Auflage," AViirzburg, 18C3. 



