432 Dr. I. Miiller on the Seminal Fluid of Tanthina. 
for carrying on such an investigation as this; and when I was 
able to resume it, I found that my whole stock of material had 
become useless in consequence of the commencement of decom- 
position. 
In the vicinity of the conical point several little lobes, resem- 
bling narrow cilia, separate from the membrane. While the 
structure is swimming, these little lobes oscillate rapidly and 
strongly, and the whole membrane is in lively undulating move- 
ment. When towed along by this singular swimming-apparatus, 
the tail always appeared to me to be perfectly quiet ; the whole 
structure, from the conical apex of the undulating membrane to 
the rounded extremity of the tail, then forms a slightly curved 
bow, and the course through which it passes follows a similar 
curve. When the membrane, and with it the head, are quiescent, 
the tail is seen slowly bending and twisting about, although 
without producing any perceptible change of place. 
Deceived by such manifold movements, I was led, in 1860, to 
regard these structures as parasitic animals, in which, however, 
I vainly endeavoured to discover traces of a mouth, intestine, &c. 
But when I was recently (1862) again able to examine a male 
Lanthina, I found my supposed parasites so densely packed in 
its semen, that I began to doubt whether I had not before me 
an essential constituent of the semen. And then I was at once 
struck with the similarity between the agitated hairs of the tail 
and seminal filaments which have nearly attained maturity, but 
have not yet separated from the place of their formation ; and I 
soon succeeded in breaking up several tails into groups of un- 
mistakeable seminal filaments, perfectly resembling those which 
were swimming about freely in the seminal fluid. 
That these structures are an essential constituent of the semen 
was consequently established. But are they the formative organs 
of the seminal filaments, from which these subsequently, when 
mature, separate? or are they “ spermatophora,” around which 
the mature seminal filaments have collected? The former notion 
appears to me the more probable one; it is supported especially 
by specimens frequently observed, in which the seminal filaments 
were not only motionless, but also appeared to be shorter than 
inthe others. Besides these, numerous other still younger forms 
were seen : the youngest that came under observation was of the 
form of an elongated egg, about 0:2 millim. in length and 
0-1 millim. in breadth. The greater part of this oval body ap- 
peared perfectly transparent and empty ; the thickened end alone 
was occupied by a roundish mass, which was rendered opaque by 
densely imbedded granules. It appeared darker on the side 
turned towards the apex of the egg, and lighter on the opposite 
side, although no distinct line of demarcation could be detected 
