LINNEAN SOCIETY OF LONDON. lxi 
the under side of the head—a most unusual position, and one for 
which we can as yet find a parallel only among the Arachnida ; 
Trombidium holosericewm having the spiracles, according to Pa- 
genstecher, at the base of the mandibles. In several other genera 
of Thysanoura, and even in the new genus of Smynthuride now de- 
scribed, Mr. Lubbock has found no trachex, and believes that 
respiration is principally effected through the general surface of 
the skin. 
How different this account of the internal anatomy is from that 
given by M. Nicolet, a glance at his figures will show. In the 
digestive organs he describes and figures, first, /’ésophage ; secondly, 
jabot ; thirdly, ventricule chylifere ; fourthly, vaisseaux hépatiques ; 
fifthly, intestin gréle; sixthly, cecwm (the last no doubt a slip of 
the pen for rectum). In the respiratory system he finds eight 
spiracles on the dorsal surface of the first four abdominal segments, 
and an elegant network of trachee. Mr. Lubbock has not indeed 
as yet met with Podwra similata—the species which M. Nicolet 
dissected ; but that author appears to extend his descriptions to the 
whole group, and has been so understood by subsequent writers. 
Even if his descriptions are correct for the species with which he 
worked, they cannot be applied to the other genera ; and the state- 
ments made in general works on comparative anatomy must 
therefore be considered as altogether erroneous. 
The so-called “ gastric tube,’”’ and the two tentacles which pro- 
ceed from it, are not the least curious part of the anatomy of 
Smynthurus. The tube itself is situated between the posterior 
legs; and the two colourless, semitransparent tentacles which 
can be projected from it are about th of an inch in length, and 
covered with gland-like papille. When creeping on a slippery 
surface the animal uses these tentacles to ensure its hold, or if 
turned over on its back, avails itself of their assistance to regain 
its normal position. Each tentacle contains two muscles, one of 
which is attached at the extremity, and the other at a point 
about halfway. These two muscles are, at their other end, attached 
to the back, close together. We might at first be inclined to wonder 
at the presence of two muscles, and to suppose that the object 
might have been equally well attained by a single one. The two, 
however, acting on different parts, may enable the organ to be 
retracted more quickly than might otherwise be the case—an ad- 
vantage doubtless of much importance in a structure so delicate. 
Again, it is evident that a single muscle attached to the end could 
LINN. PROC.—VOL. VI. Bi 
