LINNEAN SOCIETY OF LONDON. 



Ixi 



the under side of the head — a most unusual position, and one for 

 which we can as yet find a parallel only among the Araclinida ; 

 Trombidium Tiolosericeum 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 Smynthuridce now de- 

 scribed, Mr. Lubbock has found no tracheae, 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, Vesophage ; secondly, 

 jabot ; thirdly, ventricule chylifere ; fourthly, vaisseaux hepatiques ; 

 fifthly, intestin grele ; sixthly, ccecum (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 tracheae. Mr. Lubbock has not indeed 

 as yet met with Podura similata — the species w T hich 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 yVth of an inch in length, and 

 covered with gland-like papillae. 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. peoc. — vol. vi. f 



