496 MK. A. D. MICHAEL ON THE 



"Whether tlie pericibal gland of Bdella can be considered as the homologue of any 

 particular gland in Thyas, TromhicUmn, and Hijdrodroma, seems to me to be uncertain : 

 the fact that each pericibal gland of Bdella discharges by its own special duct into a 

 place different from, the point of discharge of the common duct, whereas all those of 

 Thyas and Tromhidium discharge by the common duct, is greatly against such homo- 

 logy ; but, on the other hand, the number of glands is the same in all : if there be any 

 homology it would be between the pei'icibal gland of Bdella, the quadrate salivai'y gland 

 of Thyas, and the smaller dorsal mouth-gland of Sydrodroma ; but this can only be the 

 case if the anterior salivary gland of Bdella be the homologue of the small gland to which 

 the duct " dsa " leads in Thyas. 



The Azygous Salivary Oland. — The last of the so-called salivary glands to be described 

 is the azygous (figs. 10 and 34, so, and 12). Whether this gland can be considered to be 

 tlie homologue of the " azygous salivary gland" in Thyas petrophilus, the only species in 

 which an azygous salivary gland has been described, is again a question which admits of 

 differences of opinion : the mere fact that such a gland exists in each would raise a 

 presumption that they were homologous ; but not only has the gland in Bdella an 

 importance immensely in excess of that of the gland in Thyas, but the gland of Thyas is 

 situated in the rostrum, whereas that of Bdella is in the cephalothorax behind the 

 rostrum ; that in Thyas is about the middle (dorso-ventrally) of the rostrum, whereas 

 the gland in Bdella is jiressed against the extreme dorsal surface of the cephalothorax. 

 The duct in Thyas discharges much further forward than that of Bdella ; and, finally, 

 the histology is about as different as that of two glands can well be ; the gland in Thyas 

 having a minute structure very like that of the tubular salivary glands of Bdella, while 

 the gland in Bdella is a solid, but not fleshy, organ, with a very small irregular lumen, 

 or sometimes without any distinct lumen, from the fact that its sides may become pressed 

 together, and composed of largish columnar cells, which have the ends which come to 

 the exterior of the organ slightly enlarged ; in these enlarged ends are situated the small 

 nuclei, with very distinct and dark-staining, but small, nucleoli (fig. 34j. The cell- 

 contents hardly take stain at all; the cell-walls * do not stain, but yet are very easily seen. 

 In Bdella Basferi the cells have an average length of about "04 to '05 mm., but some 

 near the posterior end are longer, by an average breadth of about 10^ to 12 /«. The 

 nuclei have an average diameter of about 5 /», and the nucleoli of under 2 in. For the 

 same species the cell-contents have the appearance of a very fine network ; whereas in 

 some of the smaller Bdellce, e. g., B. vulgaris, the gland is even more solid and has the 

 appearance of having an almost tendinous consistency (fig. 12). 



The azygous gland is a very large one ; in B. Basteri it has an average lengtli of about 

 •4 mm. in the male, and of about "5 mm. in the female, with a breadth of about '17 mm., 

 and a thickness of about '15 mm. to '19 mm. in its thickest part. In some of the smaller 

 species it is even larger in proportion. In B. Basteri the exterior of the gland, when 



* The expression " cell-wall " in this paper is not employed in the restricted sense in which it is used by many 

 modern writers on vegetable liistology, viz. as meaning formed (secreted) material only ; it is used, for want of any 

 other equivalent expression known to me, in a general sense to signify the partition between cell and cell irrespective 

 of how that partition originates or of what it is composed. 



