INTEENAL ANATOMY OF BDELLA. 509^ 



oval sac, which lias extremely thick walls and a small lumen. The greatest length of 

 the organ in B. Bastevl is usually about "2 mm. 



The histology of the organ bears a resemblance to that of the mucous glands, yet tlie 

 differences are considerable. The tunica propria is present, but is not nearly so easily 

 seen as in the mucous glands, because the cell-contents and walls in the antechamher 

 hardly stain at all, and therefore do not dilferentiate. The walls are composed of 

 columnar cells radiating inward, and in B. Basteri mostly about -04 mm. in length ; 

 but in tliat species they are very irregular in arrangement (fig. 38). In some other 

 species, e. g. B. vulgmHS, they are remarkably regular, and the walls of the cells strong 

 (fig. 41). The cell-contents are very compact and the nuclei extremely small, 

 flattened, and indistinct ; indeed it would be difiicult to see them were it not for tho 

 minute darkly staining nucleoli. In some specimens it will be found that the cells are 

 empty and the nuclei gone, but the lateral walls of the cells still persist, forming a 

 strong oj)en houeycomb-like reticulum. 



The secretion within the lumen of the antechamber is similar to that foimd in the 

 mucous gland, and probahly conies thence. 



The Benial Canal (figs. 18, 21, 34, 42, ^jc, including pc^ and -pc^) is a very 

 complicated organ, and one not at all easy fully to grasp the construction of; it 

 varies considerably in detail in different species ; it must probably be considered a 

 development of a ductus ejacidatorius. When looked at from the exterior this 

 complication is not seen ; it then appears to be, and its external coating is, a large organ 

 divided into two parts (fig. 18), an inner fleshy portion {pc^), and an outer membranous 

 portion (^J6*-). The former of these is almost perpendicidar or tends somewhat 

 backward, varying in different specimens, the dorsal end being the more anterior ; it is 

 an approach to jj^-ramidal in form, but is curved, the dorsal end curving over toward 

 the posterior end of the creature ; it also enlarges gradually, but not regularly, from the 

 dorsal to the ventral end. It is round, or Jilmost so, in transverse section cut through 

 its dorsal part ; more compressed laterally in its lower part. In some other species, e. g. 

 B. vulgaris, the dorsal end of the penial canal is enlarged and is more turned to the 

 rear. On the exterior of tliis organ near its lower edge is placed on each side a 

 chitinous sclerite (as), which I will call the "amphioid sclerite " *, the anterior half of 

 which is a plate of irregular form somewhat convex exteriorly, so that it looks semi- 

 arcuate in longitudinal section ; this plate has a pointed anterior end, where it ajjproxi- 

 mates to its fellow of the opposite side. The posterior half turns in a sigmoid 

 manner, and is thicker and rod-like ; its hind end almost touches the tube which leads 

 from the outer membranous chamber of the penial canal into the air-chamber on that 

 side of the body. The anterior plate-like half of this amphioid sclerite is bordered by 

 4 or 5 spines ; its principal function appears to be to give attachment to muscles, 

 although it probably is also protective ; from it spring the muscles (mdl) which 

 divaricate the labia that close the entrance from the azygous accessory gland 

 (gaa) to the penial canal. From the posterior dorsal angle of the plate arises a thick 

 tendinous projection, and into the end of this a pair of large retractor muscles {mr) 



* 'A/^Kfii, both ; €Uos, shape. 

 SECOND SEMES. — ZOOLOGY, VOL. VI. 69 



