IT. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 311 



to it by means of the flagellum (/) ; and the latter and the lip (lp) force it into the oral 

 aperture exactly in the same way as has been described in regard to Monas. 



The contractile vesicle (cv) is a single globular organ, which lies on the corresponding side 

 of the body with that of Monas, and just in front of the middle. In full diastole its diameter 

 equals one third of that of the body. Both the systole and diastole are very slow. 



The calyx (c) is about twice as long as the body which it encloses, and between four and 

 five times its own average diameter. It has the form of a very deep, slender urn, with a 

 rounded bottom, slightly contracted waist, and a very delicate, scarcely reverted, truncate 

 rim. It is so hyaline and faint that it almost defies any magnifying power below that of 

 eight hundred diameters. The pedicel (pd) which supports it is at least twice as long, of uni- 

 form diameter throughout, and very slender, in fact not much thicker than the flagellum. 

 It is attached (pd 1 ) to the bottom of the calyx exactly opposite to the point from which the 

 contractile ligament (V) arises, but, unlike the latter, it appears to be totally incapable of 

 contraction. 



§ 4. BlCOSCECA LACUSTRIS, 110V. Sp. 



(Plate IX., figs. 33, 33% 33 b , 33 c .) 



This species lives in quiet streams and lakes, attached to filamentous Algae, and is quite 

 common, especially on old specimens of Tfygnema. It is tinged throughout with a yellowish 

 color, which seems to add a good deal to the difficulty of distinguishing its various parts. 

 When protruded (fig. 33) it occupies the anterior half of the calyx (c) and projects a little 

 beyond its edge, and consequently its retractor ligament (r) stretches over the whole posterior 

 half of the dormitory. The shape is rather elliptical than elongate-oval, but it varies 

 more or less between these two forms, and seems to have the latter shape in the largest indi- 

 viduals. Posteriorly the body is rounded, but its broadest region is about the middle, and 

 from thence it tapers considerably to a truncate front, and ends on one side in a laterally 

 projecting flagellum (fl), and on the opposite side in a long incurved Up (lp). 



The longitudinal furrow (r 1 ) which is so conspicuous in B. gracilipes is much narrower in 

 this species and not so deep, yet it holds exactly the same relations to the base of the 

 flagellum (fl), and the contractile ligament (;•). After a number of observations upon the 

 frequent and sudden retraction of the body to the bottom of its calyx, during which in 

 every instance that side along which the furrow (fig. 33°, r 1 ) runs was contracted much 

 more than the opposite one, I feel quite confident that this sulcus is the seat of a highly 

 contractile band, and moreover that it is continuous with the posterior retractor liga- 

 ment (r). The latter is very slender and thread-like, and is attached to the posterior end 

 of the body on one side (see fig. 33", r) of its axial line, and has very much the appearance 

 of being a free continuation of a ligament in the furrow just mentioned. The lip is nearly 

 twice as long in proportion to the breadth of the front as that of B. gracilipes, and has 

 an incurved, digitate form (figs. 33, 33 a , lp). 



The flagellum (fl) is the most remarkable and distinguishing feature of this species, when 

 contrasted with B. gracilipes, on account of the wide angle at which it diverges from the 

 longitudinal axis of the body; for whilst in the latter it deviates but little from parallelism 

 -with the axial line, in the former it arises at an angle of from forty to forty-five de- 

 grees (fig. 33 %fl) with the same line. At its base it curves away from the lip, but for the 

 remaining four fifths it bends with a long arch in the opposite direction, but not so much as 



