316 
MR. WILSON ON THE STRUCTURE AND DEVELOPMENT OF 
and of an inch, and its breadth between 333 and 33-3 ; one remarkably long 
specimen measured 33-. 
The development of the short variety of the Entozoon is unattended by an inter- 
mediate embryonic form. The elements of the head and the feet are produced from 
the ovum itself, and with very little alteration in the form of the latter. I have 
already remarked that the largest of the ova become elongated, and if we suppose 
that the head is produced from one extremity, while three pairs of legs are projected 
at the sides, we shall have the young form of the animal. The smallest specimen 
which I have examined was somewhat exceeded in length by the largest of the ova, 
measuring only 3-5-3, while the ovum measured -3-33 of an inch. And I found a gra- 
dual progression in length from that specimen upwards to the more perfect kinds. 
This early form, like the analogous one of the long variety, possesses only three pairs 
of legs, which resemble mere protrusions of the external membrane. The head is 
equally imperfect in form, and the abdominal striae are not apparent ; the internal 
structure again is a mere assemblage of cells ; the changes which ensue during the 
progressive improvement of this form are, — an elongation of the abdomen, the forma- 
tion of abdominal rings, and of the thoracic markings. There is one peculiarity 
which is more remarkable in the young of the short variety than in that of the larger 
kind, and that is, the very frequent ecdysis. The three-legged form of the short 
variety measures in long diameter from 3-5-3 to 337, and in breadth from to 777 
of an inch. 
Like the long variety, the young of the shorter kind has a four-legged as well as 
a three-legged stadium, the production of an additional pair of legs becoming appa- 
rent after the throwing off' one of the exuvim. During this stage the animal begins 
to assume the general form of the perfect state ; the thorax loses its straitened cha- 
racter and becomes rounded and larger ; the abdomen becomes longer, and instead 
of tapering gradually towards the tail, maintains its bulk until near the extremity, 
when it becomes suddenly smaller, and forms an elongated point. Moreover, during 
this period the head is more fully formed; the segments of the thorax and legs are 
more completely developed, and the abdominal rings more distinct. The internal 
structure is also modified and improved. The measurements of the four-legged 
stage of the young of the short variety range in length between yyro and and in 
breadth between 337 and 3-3-3- of an inch. 
On reviewing the structure of this remarkable animal, I feel again disposed to 
revert to the position which it should occupy in the animal kingdom ; that it is not a 
member of the class Insecta seems proved by its possession of four pairs of legs ; that 
it is not an Acarus must be manifest from the preceding description ; and that it is 
not an Arachnidan appears to be equally certain. The distinct separation between 
the head and thorax, the incomplete separation of the latter from the abdomen, the 
annulate structure of the abdomen, the free movements of the latter, and especially 
its capability of contraction, both in the longitudinal and transverse direction, are 
