112 The American Spoyigilla. 



firm this, and show that it is a single fiagellum. The monads are 

 so transparent, and the organization so distinct, that the collar and 

 fiagellum may he seen clearly from an opposite point of view, look- 

 ing directly through the hody of the cephalid. This, too, is the 

 best position from \\hich to study the contractile vesicles. 



A sectional profile view of a group (Fig. 4), to he obtained 

 by plunging the focus half-way through a chamber, serves best 

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

 monads are afiixed 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 j)ower than in Fig, 2 or 4. An excellent and least 

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

 focussing upon the monads immediately about the aperture of the 

 chamber. Here we look directly into the door-way, 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 posterior 

 end is broadly rounded, and so is its anterior extremity. In front 

 rises a cylindrical, membranous "collar" (k), which tapers 

 slightly and projects forward to a distance equal to considerably 

 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 conspicuous 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 toward its basal attachment. 



The fiagellum (/) arises from the centre of the anterior end of 

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

 membranous cylinder (Jc), and without tapering extends a little 

 farther than the open end of the latter. It vibrates usually 

 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 distinctly 

 marked by a coarse, scattered, brown granulation, with two or 

 three rather large, clear spots, at a considerable distance from each 

 other, but always close to the periphery. These clear areas are 

 the contractile vesicles {v). They do not occupy any particular 

 place in the body, although they usually are not in front. The 

 systole and diastole are extremely slow, but very distinct, if sufiicient 

 patience is summoned to watch them fixedly, and without inter- 

 ruption. The last third of the systole is abrupt, and then only 



