78 Prof. H. James-Clark on the American Spongilla 



circle," the two being concentric to each other. At the same 

 time we see in the centre of the dark circle a black spot [l) 

 which may also be focused up and down upon, and hence 

 it is inferred to be a continuous line foreshortened. Other views 

 (fig. 3, T) confirm this, and show that it is a single Jlagellum. 

 The monads are so transparent, and the organization so di- 

 stinct, that the collar and fiagellum may be seen clearly from 

 an opposite point of view, looking directly through the body 

 of the cephalid. This, too, is the best position from which to 

 study the contractile vesicles. 



A sectional profile view of a group (fig. 4) , to be obtained 

 by plunging the focus halfway through a chamber, serves best 

 to disclose the manner in which the posterior ends [j) of the 

 monads are affixed to the concave face of their receptacle ; and 

 we also here obtain a strictly profile aspect of a monad. Figure 

 3 is such a view, representing a single cephalid under a much 

 higher power than in figures 2 or 4. An excellent and least- 

 obstructed side view, but not strictly a profile, is to be had by 

 focusing upon the monads immediately about the aperture of 

 the chamber. Here we look directly into the doorway, or 

 through the bordering transparent epithelioid membrane which 

 it penetrates. 



The body proper (fig. 3,j) of a cephalid is a little shorter 

 than it is broad, on the whole spheroidal in shape. Its pos- 

 terior end is broadly rounded ; and so is its anterior extremity. 

 In front arises a cylindrical membranous " collar " (k), which 

 tapers slightly and projects forward to a distance equal to con- 

 siderably more than twice the length of the body. Its diameter 

 is not more than two thirds, or even less than that, of the body. 

 Although colourless and homogeneous, it is remarkably con- 

 spicuousj on account of the thickness of the membrane of which 

 it is composed. Near its open extremity it is more transparent 

 and less obvious than towards its basal attachment. 



The Jlagellum (1) arises from the centre of the anterior end of 

 the body, in the midst of the area which is surrounded by the 

 membranous cylinder {k), and, without tapering, extends a 

 little further than the open end of the latter. It vibrates usu- 

 ally throughout its length, but is most active near its tip. We 

 have never seen it assume a rigid, arcuate position, as in some 

 other species of monads. It is particularly remarkable for its 

 want of transparency, and looks like a black thread more than 

 any vibrating cilium that we have ever met with. Its action, 

 at times, is rather that of a strong wriggle than a vibration. 



The contractile vesicles {v). — The body of the monad is di- 

 stinctly marked by a coarse, scattered, brown granulation, with 

 two or three rather large clear spots at a considerable distance 



