Sliidii's uii iii.n-iiir ()striii(iils 91 



such characters as distinguish a cleaning limb from a crawling leg (cf. G. \\. MUllku, 1894, 

 pi. 12, fig. 49). If we turn to the family DarwinuUdae, which is closely related to the 

 C y p r i d s, we find that although the fifth limb has been developed as a presumably rather 

 effective masticatory organ, the seventh limb is not differentiated as a cleaning organ but has 

 entered the service of locomotion. Nor do the conditions in the C y p r i d i n i d s support 

 the assumption put forward by G. Alm. The fifth limb in, for instance, the genera Cypridina 

 and Philomedes certainly helps considerably in breaking up the food, even to a far greater 

 extent than in some C y p r i d s ; this idea is supported by the extremely powerful musculature 

 and armature of this appendage. As in these genera the seventh limb is developed as a presum- 

 ably effective cleaning organ one would think, of course, that the conditions in these genera 

 support G. Alm's hypothesis, but, as we know, the fact is that in these genera the mandible 

 does not at all help or at any rate only helps very slightly in the breaking up of the food, wliich 

 (|uite makes up for the development of the fifth limb as a masticatory organ. The Asteropids 

 are characterized by a method of taking up the food that is quite unlike that of other C y p r i- 

 fl i n i d s. As we know, a rather strong current of water from front to back is produced in the 

 C y p r i d i n i d s by the movements of the vibratory plate on the fifth limb; this is for respi- 

 ration — as is generally assumed and appears very probable. \^Tiile in most C y p r i d i n i d s 

 this stream is allowed to pass freely along between the shell and the body without losing any 

 f)f the organic and inorganic little particles that naturally accompany it, whirled up from the 

 bottom, this is not the case in the Asteropids. As has been described in another place 

 in this treatise, the limbs of the mouth have been differentiated in a very strange way in these 

 forms. The maxilla has been developed into a sort of baleen-like organ, which, with its epipodial 

 appendage and its long, fine ventral bristles fills the anterior opening of the canal through 

 which the respiratory water has to pass. By means of these baleens the water that runs througli 

 is cleaned of a great many of the defiling particles; a number of these particles constitute the 

 food of these forms. The water that, after passing the maxilla, continues backwards between 

 the shell and the wall of the body, is thus presumably much cleaner than the respiratory water 

 in other Cypridinids. It is true that the fifth limb is developed as a mouth organ in these 

 forms, but it does not act as a masticatory appendage and thus does not increase the number 

 of the defiling particles. The food is not broken up at all and the respiratory water that passes 

 is cleaned from small defiling particles before it penetrates into the part that is cleaned by 

 the seventh limb. In spite of this this limb is well developed as a cleaning organ in 

 these forms. The Halo cypr ids have in their mandible and maxilla quite as powerful 

 masticatory organs as any representative of the family Cypridae; in addition they have 

 a rather powerful masticatory part on the fifth limb. In spite of this their cleaning 

 limb is very much reduced; cf. the remark above, p. 89. Finally it ought to be noted 

 that among the families whose fifth and seventh limbs are developed as typical crawling 

 legs there are certainly forms that have more powerful and more intensive mastication 

 than a number of forms whose seventh limb is developed as a cleaning organ and whose 

 fifth limb helps more or less in intensifying the mastication. The methods by which 

 the y t h e r e 1 1 i d s and the P o 1 y c o p i d s take up their food are too little known 



