Chap. II. AURELIA FLAVIDULA. 19 



oftentimes be observed when here and tliere one or several project for a -while 

 at a different angle from the rest, and then flill back to the common inclination, 

 Avhilst others rise up or subside in the like manner at different points of the body. 

 The trend of the cilia depends upon the velocity of the body as it bores its way 

 through the water: when going swiftly, the cilia point obliquely backwards, at an 

 angle of thirty or forty degrees to the longer axis; but when progressing slowly, 

 they either vibrate with much less rapidity, or else, keeping up the energy of 

 their motions, they assume a trend more nearly at right angles to the axis of 

 revolution, and thus the body rotates very fast, without, however, advancing at a 

 corresponding rate. Thus oftentimes we may see the embryo progressing very 

 rapidly, and all at once almost or altogether cease its forward motion, without 

 retarding the velocity of its rotation. 



In the next phase {Figs. 5 and &") the body is elongate cylindrical, and, being 

 more active tlian in the last stage, the motions forward and backward, and the 

 rotations and retroversions, excite .the attention more readily. There is another 

 mode of progress sometimes adopted by the embryo, which reminds one of the 

 movements of certain forms of the so-called Infusoria, such as Leucophrys and 

 Paramecium : we refer to its unaccountable habit of whirling over, end for end, 

 as a club does when hurled through the air. This it will do occasionally without 

 moving from the spot, and so persistently and rapidly that the eye sees hardly 

 any thing but a flitting shadow. The outer layers of cells are very clear, and have 

 a crystalline brilliancy, which would seem to result from the sharply polygonal form 

 of the cells ; the interior of the body is Avholly opaque, and colored deep orange 

 red. It would seem from this, that the clear interior mass had become totally 

 changed into pigment cells ; but of this we cannot speak decidedly, since the animal 

 has powers of contraction so great that it is possible the clear centre is reduced 

 to a very small size, and hidden from view by the opacity of the pigment cells. 

 Fig. 6, compared with Fig. 5, is an examj^le of the variation in size which the 

 embryos exhibit at this age. 



As a further step in development the embryo becomes oval in outline, and a 

 hollow space appears in the interior, near one end {Fig. 7 d). In the numerous 

 embryos which we have examined, this space has always appeared at that part of 

 the body which is behind when the animal swims; yet it may vary in its position, 

 as occurs in a later stage, when the whole of one end of the embryo is hollowed 

 out so as to leave a remarkably clear space {Fig. 8 d). This space, as in the 

 last stage, is usually seen behind; but occasionally the animal shifts, as it were, 

 its opaque load of orange red pigment to the opposite end. Whether the orange 

 mass within is really loosened from the outer transparent layer, or the embryo 

 has the power of suddenly forming a hollow space where it j)leases, we cannot 



