356 



MR. G. S. BRADY'S MONOGRAPH OF 



of a special sense (smell or hearing), being clotlied with toothed club-shaped appendages, 

 which either proceed directly from the antennae ( Conchoecia), or from its bristle-shaped 

 appendages ( Cypridina). The lower antennse consist of a particularly large and muscular 

 basal joint and two branches, one of which may be rudimentary, whilst the other is 

 cylindrical, flexible, and composed of numerous short joints, each of which bears generally 

 on one side a long ciliated swimming-seta. The way in which these antennse move is 

 also quite peculiar. Whilst those of the previous group move upwards and downwards, 

 these move from side to side. To allow of this movement the valves are peculiarly 

 formed, so that when quite closed there is always an opening (mostly heart-shaped) 

 through which the movements of the antennse take place. In conformity with the 

 development of these antennae as locomotive organs, we find that the other limbs more 

 and more lose their importance as su'cli, and become subservient to the mouth as jaws. 

 In Conchoecia the two hinder pairs are somewhat pediform ; but their use as feet must 

 be but small, as they can scarcely reach out of the valves. In Cyiwidina and the allied 

 forms, the three pairs of limbs following the mouth are completely jaw-shaped, and the 

 fourth or last pair, as in the Cypridse, is bent up within the valves. The structure of 

 this limb is so peculiar that one can scarcely class it as a foot or a jaw. The first pair 

 of jaws possesses no branchial plate, which is constantly present in the foregoing group, 

 but this appendage is transferred to the second pair {Cyprid'ma), or the second and 

 third pairs {Conchoecia). The postabdomen is developed into two broad symmetrical 

 plates, armed with sharp claws, and is not unlike the same organ in the Cladocera. The 

 eyes, when present, are fixed, as in the Branchiopoda, to large separate peduncles. In 

 Conchoecia there are, at the roots of the upper antennse, certain small bodies which take 

 the place of eyes. In this group appears for the first time an independent heart, of 

 structure similar to that of the Branchiopoda. The digestive canal has a single capacious 

 stomach, which is invested throughout its whole length by a thick layer of liver-cells. 

 Most of the forms belonging to this group are pelagic, and occur often in the open sea, 

 near the surface of the water, where they move actively by means of the largely deve- 

 loped lower antennse. Many of them are strongly phosphorescent in the dark, and must 

 contribute essentially to the extraordinary brilliance of the tropical seas. 



3. Cladocopa. — The type of this group is Polycope, a genus recently described by 

 G. O. Sars, which occurs on some parts of the coasts of the British islands and Nor- 

 way, and in the Mediterranean. The lower antennse are very similar to those of the 

 foregoing group, but are divided into two distinct and well-developed natatory branches. 

 The movements of these organs are quite different. Whilst in the Cypridinidse their 

 motion is from side to side, it is here upwards and downwards as in the Cypridse ; 

 and, in conformity with this, we find that the characteristic notch of the former family 

 is absent. Eor these reasons it is necessary that the upper antennse also should become 

 natatory organs in order to bring the movement into equilibrium. The most curious 

 point in these animals, however, is that behind the mouth we find two pairs of limbs, 

 the last of which is used entirely for respiration, whilst the first pair or its palp is deve- 

 loped into a powerful two-branched swimming-organ. We have therefore, in this case, 

 a group provided with no less than six swimming-organs ; and on this peculiarity the 



