190 LEUCKART, ON PENTASTOMUM TAENIOIDES. 
tuting the mouth, the chitmous margin of which, however, 
does not thicken till a later period, when the organs of 
support become visible on the legs. 
Of late it has been the fashion to regard the Pentasto- 
mata as belongmg to the Acarina or Crustacea. But 
the above observations respecting their embryonic develop- 
ment would scarcely be in favour of this hypothesis, and 
might perhaps be adduced against it. It has hitherto been 
considered a general law, that the Arthropoda, to which the 
Acari and Crustacea belong, are developed by means of a 
primitive streak, while the Pentastomata present us with 
the mstance of a universal development, such as is usually 
observed in the Annelida. That the formation of the embry- 
onal legs is also by no means to such an extent mite-like or 
crustacean, as was formerly assumed when an articulation 
was assigned to them, has been demonstrated above in detail. 
According to the present state of our knowledge respecting 
the Pentastomata, it would scarcely be at once considered 
erroneous and unnatural, if some one undertook to defend 
the old view of the worm-nature of these animals. Although 
a further and more exact imvestigation of these relations 
would be here inappropriate, I have referred to them because 
many modern helminthologists, well known to the medical 
public, have probably set too much weight on the presumed 
crustacean and acarime character of these parasites. Again, 
on the other hand, notwithstanding the doubts I have here 
expressed, we must not forget, that im various respects (such 
as the general form of the embryo, the formation of the 
chitinous covering, and transverse striation of the muscles, 
&e.) the Pentastomata do really exhibit a greater approxima- 
tion to the higher articulated animals, than we are elsewhere 
accustomed to see in a worm. 
After these remarks let us return to our proper subject, 
z.é., to the life-history of the Pentastomata. 
The three dogs into which I had introduced the Pentastomum 
denticulatum were thus, on examination, without exception 
found to be infested with P. tenioides, and some of them 
even with very numerous individuals of that rare parasite. 
It was also apparent that the various states of development of 
these animals corresponded accurately with the differences 
and duration of the experiments—that is to say, their deve- 
lopment was always the more complete, the longer the 
interval since the period of introduction. 
From these results we are fully justified in concluding that 
the Pentastomata found in the infected-dogs were descended- 
from the introduced P. denticulatum. In other words, that 
