764 
MR. H. N. MOSELEY ON THE STRUCTURE AND 
at all, and, in consequence of the great imperfection of their spiral fibre, are with great 
difficulty to be made out, even with a high power, under the microscope. It is thus 
easy to understand how they escaped the observation of Grube. The tracheae arise 
from the skin all over the surface of the body, but are specially developed in certain 
situations. When the animal is opened under water in the fresh state, the groups of 
them appear to the naked eye as pearly white dots scattered over the inner surface of 
the body-wall, and, when viewed with a lens, show themselves as small tree-like rami- 
fications coming up through the tissue and ramifying on the surface. These are shown 
in Plate LXXII. fig. 2, where they are represented as dark on a light ground for con- 
venience of drawing : in reality they appear glistening and pearly. 
Especially large and conspicuous groups of tracheae are supplied to the rectum and 
pharynx (Plate LXXII. fig. 1, tr, tr). These tracheae arise from the middle line of the 
ventral surface. 
The structure of the tracheal system resembles very much that of lulus. Plate 
LXXIII. fig. 5 shows the mode of origin of the tracheae from the skin-surface. A 
short wide tube leads from a simple opening between the cells of the epidermis directly 
inwards through the skin. The tube is contracted at its opening, and swells out some- 
what at its inward termination. From the inner end of this tube arise all together a 
vast number of fine tracheal vessels in a sort of brush. The vessels run for the first 
part of their course in a densely packed cylindrical mass, apparently united together 
side by side, running parallel, hut never communicating. The cylindrical mass of 
vessels soon divides into several trunks, which again subdivide, and so give off the 
bundles of vessels which are distributed to the viscera, &c. The tubes never ana- 
stomose in any part of their course. In the first part of their course, where they are 
closely packed together, they run in almost straight lines ; but as soon as they pass into 
smaller bundles, or are distributed to their destinations, they take a tortuous or zigzag 
course (Plate LXXIII. fig. 2). The closely packed masses of trachese at their origin, 
when filled with air and viewed by transmitted light under the microscope, are very 
conspicuous objects, and appear quite black, transmitting no light. In order to see the 
mode of origin of the trachese well, a vertical section of the body-wall of a Peripatus 
must be made in the fresh condition, and placed in glycerine for examination. It 
should be rather thick, or otherwise most of the air-vessels are cut across, the air 
escapes, and the vessels become almost invisible. When a thick section is thus 
prepared, and pressure is brought to bear on the covering-glass, several small bubbles 
of air may be caused to pass out from the spiracular opening of one of the tracheal 
bundles. Sections prepared from specimens hardened in spirit show almost nothing 
of the tracheal system, and it is hence difficult to get transverse sections to show the 
structure of the primary short air-tube. It must, however, terminate internally in a 
finely perforated plate. Some few tracheal vessels are given off at once from the large 
primary mass to the immediately surrounding tissue ; but the larger part run a long 
course in parallel bundles before they separate for their final disposition. All the 
