PHYSOPHORA. 83 



The structure of this involucruin differs from that of the corresponding part in 

 Stephanomia and Agnlma, inasmuch as it is only partially formed by a lamellar expansion 

 of the ectocyst ; the greater part of its wall being constituted by a cup-shaped dilatation of 

 the pedicle itself, from whose edges the proper involucrum rises. As a consequence 

 of this enlargement of the pedicle, the attached end of the sacculus, distinguishable at 

 once by its large oval thread-cells, has been carried to the distal extremity of the involucrum, 

 instead of remaining at its proximal end. And at the same time, as the sacculus has 

 increased in length far beyond the diameter of the sac, it has become coiled into a 

 helix with several close turns. By pressure it can be extruded from the involucrum. 



As I have stated, none of those organs which I examined were fully formed. Indeed, 

 they must have been far removed from their adult condition, for they were not more than 

 one sixtieth of an inch in diameter; while, according to Philippi and Kolliker, the fully 

 formed sacs measure a line or a line and a half. Both Kolliker and Vogt, however, figure 

 and describe intermediate states, which completely bridge over the interval between the adult 

 forms which they more particularly investigated, and the very young ones of which I 

 have just given an account, but of which they say nothing. 



Vogt's pi. V, fig. 10, and Kolliker's pi. v, fig. 3, represent successive intermediate stages ; 

 Vogt's being but little more advanced than my oldest. The sacculus here makes seven 

 or eight turns, and its proximal end is represented as lying free, while the distal end, 

 containing many large oval thread-cells, is attached to the wall of the involucrum. 



Kolliker's figure represents a rather more advanced stage, the distal extremity of 

 the involucrum being produced into a point ; otherwise, so far as the figure is concerned, 

 I should have said there was no essential change. His description, however, presents 

 difficulties. 



" These (organs) consist of an oval vesicle produced into a point at its free end, and whose 

 thick granular walls, composed apparently of small cells, inclose an oval cavity (provided with 

 a small aperture near the pedicle), which contains a clear fluid and a large spirally coiled 

 filament. This commences at the upper (distal) end of the investing capsule, with a rounded 

 free end, runs down, making six or seven turns, towards the pedicle, and ends near the 

 aperture in a narrow cord, which ascends through the cavity of the spiral, and is inserted 

 into the upper (distal) end of the capsule. On examination the spiral thread turns out to be 

 a cylinder full of thread-cells, and the cord to be a muscle, which, divided into two bands, is 

 continued on to the spiral thread, and runs along its concave side to its free end" (p. 22). 



From this description it will be observed that the general structure of the organ appears 

 to be the same as that which I have described, except that the spiral is turned round, its free 

 end being distal instead of proximal, and the proximal end remaining connected with the 

 original place of attachment by a muscular band. 



Such a bouleverscment is almost inconceivable, and Kolliker's fig. 2, and Vogt's figs. 9 

 and 1 1 , convince me that it really has not taken place, for in all these I find the end of the 

 spiral cord, which contains the large oval thread-cells, distal, i. e., in the same position as in 

 my figures. 



I conceive, in fact, that the real state of the case is this. As the sacculus enlarges, a 

 strong band of muscular fibres is developed within its thin wall, and it is the edge of this 

 band which, in the retracted state, appears like a distinct muscular cord traversing the axis of 



