446 Prof. Leuckart on a Sucker-like Apparatus 



which it bore upon its back at some distance from the anterior 

 end of the shell. This had the form of a plate-like pit, with a 

 swelled margin and distinct muscular structure, annular fibres 

 in the periphery, and radiating fibres in the middle. Even if the 

 structure of this organ could have left any doubt as to its func- 

 tion, this could not but disappear when I saw the animal attach 

 itself to the side of the glass by means of this apparatus. 



The organ in question has already been repeatedly seen by 

 previous observers, and must occur pretty frequently in the 

 allied animals, although perhaps rarely so perfectly developed. 

 Nevertheless, its signification has hardly yet been recognized, 

 a circumstance which may perhaps partly be due to the fact that ' 

 sucker-like adhesive organs usually occur only amongst para- 

 sites, and are almost wholly wanting amongst the Arthropoda. 



Amongst those naturalists who observed this structure before 

 me, I may especially mention Loven, who describes it in Evadne 

 Nordmanni as a " circular muscle " attached to a depression in 

 the shell, and consisting of radiating fibres. Loven regards this 

 " muscle " as a part of the ordinary cutaneous layer of muscles, 

 without referring further to its peculiarities or indicating its 

 function. Liljeborg saw the same sucker in Polyphemus (De 

 Crustaceis ex ordin. trib., 1853, tab. 5. fig. 3), but regarded it, 

 singularly enough, as an organ of secretion. 



The only observer who, as far as I know, had a correct notion 

 of the organ in question is Strauss-Durckheim, who describes 

 (Museum Senckenberg. 1837, ii. p. 126) a "head-lobe" (Kopf- 

 zapferi) as a characteristic organ, previously overlooked, in Lim- 

 nadia, " by means of which these animals can adhere." 



The faculty of attaching themselves by the neck to foreign 

 objects occasionally, however, is well known to be possessed by 

 other allied Entomostraca. Even 0. F. Miiller mentions, in his 

 work upon the Entomostraca, that he has often seen Sida cry- 

 stallina in this position, with its head hanging down ; and the 

 same thing has been stated by subsequent observers, although 

 Zaddach (Synops. Pruss. Crustac. Prodrom. 1844, p. 26) admits 

 that he does not know by what organ an adhesion of this kind 

 observed by him in certain species of Daphnia and Lynceus can 

 be effected. 



When we have once made acquaintance with the sucking-disk 

 of Evadne, it is not difficult to discover, even in the other ani- 

 mals, and especially in Sida, a flattened, more or less projecting 

 tubercle in the region of the neck, and to recognize this as an 

 adhesive apparatus, although the muscularity is much less di- 

 stinct, and may perhaps differ in its arrangement from that 

 previously described. 



The existence of this dorsal sucker is, however, interesting, 



