xlvi 



complex organs differs fundamentally from that given by 

 Grenadier. Grenadier, in a subsequent memoir,* defends his 

 own views, and denies the accuracy of Graber's observations. 

 He points out that, while Scolopendra has only four eyes, the 

 number increases in Juhis, Lithuhiiis, and Glomeris ; and that in 

 Scutigera it is so great (amounting to several hundred), and the 

 facets are so close to one another, that the eye has all the 

 appearance of the compound eye of a true insect ; and if each 

 cornea throws an image on a retina, we have the difficulties 

 which Miiller has pointed out in the case of insects. 



Moreover, though the ocellus of a spider at first sight closely 

 resembles the eye of a Scolopendra, the internal structure is, 

 according to Grenadier, altogether different. In the ocellus of a 

 spider or an insect we find, at a greater or less distance behind 

 the lens, a retina consisting of a receptive surface, extended con- 

 centrically with that of the lens, and consisting of a number of 

 more or less rod-like perceptive elements so arranged that their 

 light could fall on their ends. 



On the contrary, in the eyes of Myriapods there is, he says, 

 either a single receptive element behind the cornea, or where 

 there are many they are arranged with their longer axes 

 perpendicular to the direction of the light ; so that any separate 

 perception of the rays of light coming from different points seems 

 to be an impossibility. In the eye of Lithohius, behind the biconvex 

 lens, he states that the cells, lining what I may call the tube of 

 each separate eye, terminate in hairs, between the free ends of 

 which is left a narrow passage down which the light must pass to 

 reach the end of the optic nerve. Such a structure, certainly very 

 remarkable, seems entirely to preclude the possibility of the forma- 

 tion of a true image. Altogether the account given by Grenadier, 

 both as to the mode of action of the eyes of Myriapods and as to 

 their internal structure, differs entirely from that of Graber. 



Sograff, also, has recently given a description of the eye of 

 Lithohius, but as his memoir is in Eussian I am unfortunately 

 unable to state his views. 



Mr. Hammond has published, in the ' Linnean Journal,' an 



interesting paper on the structure of the thorax of the Blow-fly. 



He concludes that the thorax of Diptera is almost exclusively 



mesothoracic. He also expresses the opinion that where a 



* Ar. f. Mic. Auat,, 1880, xvii., p. 415, 



