514 MR. A. D. MICHAEL ON THE 



smooth, but its outer surface rises in a series of" small irregular convexities; where two- 

 convexities meet at their edges they form a short point directed inward, and from eacli 

 of these points a strong spine projects far into tlie chamber ; all these spines are centri- 

 petal, pointing toward the middle of the chamber. 



Each air-chamber is (in B. Basteri) almost entirely sunk in the fleshy wall of the 

 erabeddiug-sac on that side of the body, only a short portion of the smaller end of the 

 air-chamber projecting from the embeddiug-sac ; part of the larger rounded end of the 

 chamber occasionally projects a little into the lumen of the embedding-sac. The air- 

 chamber does not j)enetrate the wall of the embedding-sac at right angles, but in an 

 oblique direction, so that its length lies more parallel to the surface of the embedding- 

 sac than perj)eudicular to it. In the other species of Bdella which I have examined 

 there are not any embedding-sacs, and therefore the air-chambers cannot be sunk in 

 them ; but in Bdella capillata, Kramer *, the air-chamber, although very much smaller 

 proportionately tliau iu B. Basteri, and standing free without being sunk in anything, is 

 otherwise a similar organ to that in B. Basteri. In B. vulgaris the organ exists but is 

 far more rudimentary, being merely a fold of flexible membrane, forming a small 

 chamber without spines. 



In B. Basteri a narrow passage, very strongly chitinized on one side, less so on the 

 other {acp), leads out of the anterior side of the air-chamber, about one-thii-d or one- 

 quarter of its length from the smaller end ; this passage communicates with the j)ortion 

 of the penial canal ventral to the amphioid sclerite — that is to say, with the outer fold 

 of the penial canal immediately adjoining the external labia of the genital aperture. 



No solid contents are ever found in the air-chamber ; but if a living or freshly-kiUed 

 specimen be sunk in glycerine and examined immediately it will always be seen that 

 these chambers are filled with air. 



The only analogy or homology which I can suggest for these organs is a possible one 

 to the curious so-called " ram's-horn organs " found beneath the genital operculum in 

 Chelifer ca)wroides and other forms. 



The External Labia of the Genital Opeuing. — These organs (figs. 18, 19, 34, le) are large, 

 rounded, fleshy labia with a longitudinal opening, and form an almost hemispherical 

 projection on the ventral surface of the creature ; they have considerable thickness, and 

 where they meet in the median line each has a perfectly flat and smooth, somewhat 

 chitinized edge ; forming, in fact, a curved chitiuous plate on edge : these two plates lie 

 against each other so closely that it is often difficult to see the separation in sections ; the 

 labia, however, are easily separated with a hair, and then by pushing them open the flat 

 edge becomes ventral. Each labium is bordered by a row of hairs. 



The Spermatozoa. — These, when forming in the testes, are minute, almost spherical 



* My specimens agree uxactly with Kramer's description of his BdeUa ciq/dlata (13); they do not, however, 

 (luite agree with the description and figures which Berle8e(i) gives of what he considers to be this species. In 

 Kramer's description the second joint ol the palpus is rather longer than the fifth, and the proportion of the third to 

 the fourth joint is as 4 to 7. In Bcrlese's the fifth joint is longer than the second, and the third than the fourth, 

 and there are too many hairs on the fifth joint for Kramer's description. In all these points my specimens agree 

 ■with Kramer's. 



