No. 2.] ANATOMY OF BDELLODRILUS ILLUMINATUS. 509 



and perhaps limited to two small bands. Muscle fibres of the 

 former type, but with the addition of an external granular 

 mass, in which the nucleus is imbedded, about the middle of 

 their lengths, are most frequently met with. Figs. 7, 8, and 

 1 1 show a few examples. 



The inter-segmental septa are composed of more or less 

 parallel dorso-ventral muscles, which form thin sheets, arising 

 by one or two roots, from the ends of the longitudinal fibres. 

 Where best developed the component fibres are bound together 

 by cross-slips, much as are the circular body muscles. Septa 

 are aborted between the first four somites, which freely com- 

 municate; but remnants remain as a few delicate slips which 

 guy the alimentary tract to the body walls. They are well 

 developed between the sexual somites. 



The musculature of the posterior sucker is complex, and well 

 adapted to secure strength and mobility. The circular muscles 

 undergo little change ; but the longitudinal split up, by the 

 branching of individual fibres, into a set which are the direct 

 continuation of the body longitudinal fibres, a second set which 

 pass dorso-ventrally across the body cavity, a third which radiate 

 to the margins of the disc, and, lastly, a highly branched set 

 which have become slightly displaced at their posterior ends, 

 right or left from their original longitudinal direction, and con- 

 seVently pass with a slight spiral turn from the body walls to 

 the periphery of the sucker, where they cross and interlace 

 with their fellows having an opposite displacement. 



Alimentary canal.— Tht digestive tract is distinguishable 

 into oral, pharyngeal, oesophageal, intestinal, and anal regions. 

 The mouth has an extreme anterior position between the 

 very mobile lips into which the peristomial annulus is divided. 

 The dorsal lip is slightly the longer, and, projecting somewhat, 

 gives to the mouth a slightly ventral aspect (Fig. 9). Labial 

 papillae, so well developed in B. philadelphicus and in Branchi- 

 obdella, are absent, a few faint ridges indicating a slight ten- 

 dency toward their development. Sensory hairs are also absent. 

 Circular muscle fibres continue unchanged into the lips to the 

 oral invagination; while the longitudinal muscles partly con- 

 tinue, but mainly break up into delicate dorso-ventral and radiat- 



