210 Dr. L. Plate on Asellicola digitata. 



of the latter settle themselves close together upon the border 

 of the branchial lamina, and, indeed, cliiefly upon the parts 

 which are situated outwards and posteriorly. The inward- 

 directed side of the edge, as well as the surface of the lamina, 

 is only exceptionally inhabited, usually only when the outer 

 margin is already densely covered. 1 have also frequently 

 observed on branchige which displayed very numerous Aselli- 

 colce seated close together that the few specimens on the inner 

 border were all smaller, i. e. younger, than the others, so that 

 it may be assumed that the swarm-buds of our animalcule, 

 notwithstanding their want of external sense-organs, are very 

 well able to distinguish the different surfaces of the laminae 

 and only content themselves with the inner margins when 

 the outer ones are overfull. This evidently purpose-like 

 mode of procedure in so lowly an organism cannot very well 

 be regarded as purely mechanical. It is true that in conse- 

 quence of the position and movement of the gill-lam inse the 

 flow of water in both Crustaceans passes over these organs of 

 respiration from without inwards, and the marked preference 

 of the Asellicola for the outer margin is readily intelligible on 

 account of the more abundant supply of food which is here 

 presented to it. But, on the other hand, the swarm-buds of 

 Dendrocometes are in exactly the same case, and yet they 

 attach themselves indifferently to any part of the surface of 

 the lamina, but carefully avoid the margin, as if they knew 

 beforehand that the basal membrane * of the developed animal 

 can only find a suitable support upon a flat surface. 



B^irocliona gemmipara^ Stein, also quite unmistakably 

 prefers the outer margins of the laminae, of which I have 

 convinced myself by an examination of my permanent prepa- 

 rations, and it also agrees with Asellicola in that specimens 

 but seldom pass on to the peripheral surfaces of the laminse. 

 We must therefore ascribe to the swarm-buds of the three 

 gill-frequenters here referred to the faculty of distinguishing 

 those parts of the laminse which best suit their species in the 

 adult state from those which are less favourable. In conse- 

 quence of this instinct we find the young Asellicolce frequently 

 seated so close together that subsequently, when they have 

 grown to their lull size, they have not room enough, and by 

 reciprocal pressure give each other a quadrangular form, or 

 drive a weaker individual from the margin on to the surface 

 of the lamina. 



The plasma of the cell-body does not, as in many Acinetse, 



* L. Plate, " Untersucliungen einiger an den Kiemenblattern des 

 Gcnnmanis pulex lebenden Ektoparai<iten," in ZeitscLr. f. \viss. Zool. 

 Bd. xliii. p. 170. 



