No. 2.] ANATOMY OF BDELLODRILUS ILLUMTNATUS. 507 



Mtiscular system. — The muscles of the body walls are dis- 

 posed in two layers, the outer of circular, the inner of longi- 

 tudinal fibres (Figs. 7 and 11). Both consist of elements of 

 somewhat complex structure, which has been described by 

 Voigt for Branchiobdella ; those of this species differ but 

 slightly from Voigt's description, though they are smaller than 

 in any Discodrilid which I have examined, the circular fibres 

 having a diameter of about .0075 mm., and the longitudinal of 

 .0075-.015 mm. 



Only a general account of the muscles can be presented 

 here, as a complete description of their distribution and varia- 

 tions in the different regions of the body would require the 

 space of a separate memoir. 



The circular muscle fibres do not form a continuous layer in 

 the walls of the body somites, but are scattered singly at regu- 

 lar intervals, encircling the body like hoops, of which there 

 are from 12 to 16 per somite. As before described, they are 

 closely associated with the epidermis, in which they are deeply 

 imbedded; and thus divide it into alternating cellular and non- 

 cellular zones. This relation is so intimate that it is more 

 convenient, if less accurate, to speak of a musculo-epithelial 

 layer, rather than of a muscular and an epidermal. This is the 

 more so because of the wide inter-muscular spaces which sepa- 

 rate the circular from the longitudinal muscle layers; and per- 

 mit to each greater freedom of adjustment. These spaces exist 

 principally in the major annuli of all the complete somites, and 

 extend completely around the body, except where interrupted 

 by the spermatheca and atrium, in the fifth and sixth somites, 

 respectively. In contraction the skin (epidermis and circular 

 muscles) rises freely all around in a prominent fold, like an 

 arch, as seen in optical or actual section, of which the longitu- 

 dinal muscles form the cord. In extension, the two layers 

 approach and touch, obliterating for the time the space be- 

 tween. The spaces are in part occupied by the smaller skin 

 and clitellar glands, by the terminal trunks of the nephridia, 

 the nephridial vesicle, and portions of the ductules of the great 

 lateral mucous glands, and are further, more or less, filled by 

 a loose reticulum of connective tissue fibres and cells. The 



