BRANCHIOPODA. 201 



frontal surface and become transformed into the so-called cephalic 

 horns, varying in the different species. At the same time, a sensory 

 organ (Claus, No. 21) develops between the forehead and median 

 eye ; this must not be confounded with the paired frontal organ. 

 It consists of an accumulation of club-shaped terminal nerve-cells 

 which contain peculiar three -pronged, strongly refractive deposits. 

 It corresponds to the nuchal organ described by Leydig in con- 

 nection with the Cladocera. 



The development of the Estheridae also does not deviate in any 

 essential point from that described for Apns and Branchipus. The 

 so-called Nauplius stage (Fig. 94) is still entirely devoid of the shell- 

 fold which, in the adult JEstheria, is bivalve. It is distinguished 

 from the corresponding stage of Branchipus only by the remarkably 

 large upper lip (ol) and the rudimentary condition of the first 

 antenna (a'), which is a hemispherical swelling provided with a long 

 seta (Ficker, No. 22). At a later stage, the two maxillary segments 

 and six distinctly separate limb-segments, still, however, quite devoid 

 of their limb-rudiments, can be recognised. At the posterior end 

 of the body, the furcal processes have already developed. The rudi- 

 ments of the paired eyes and of the ganglia belonging to them can 

 be made out. At this stage the rudiment of the shell first appears 

 as a paired outgrowth of the dorsal integument of the maxillary 

 region (Fig. 96, s). Within this rudiment can be recognised the 

 as yet slightly developed shell-gland. The later stages are chiefly 

 characterised by the growth of the shell, the increase in number of 

 segments and limbs, the changes which take place in the Nauplius 

 limbs, and the gradual diminution of the upper lip. In these stages 

 (Fig. 95), the larva strikingly resembles an adult Cladoceran. The 

 head, which is not yet covered by the shell-valves, has a pair of small 

 antennules beset with olfactory filaments (a'), and large swimming 

 antennae (a") recalling in a striking manner by their structure the 

 corresponding limbs in the Daphnidae. The mandibular palp (md) 

 is much reduced. The shape of the shell-gland (sd), and the form of 

 the posterior section of the body (post-abdomen), with its furcal 

 hooks and two large sensory setae, so strikingly recall the Cladocera 

 that we cannot doubt that the latter group has been derived from 

 the Estheridae by a diminution of the number of segments and 

 limbs, and a degeneration of the heart to a short sac (Claus, No. 8). 



In its larval development, Limnadia closely resembles Estheria 

 (Lereboullet, No. 26). Here also the two shell-valves develop as 

 originally distinct outgrowths of the dorsal integument of the 



