304 APPENDIX IV 



visible in all the Apodidse. We originally thought that 

 this spot was the remains of a frontal cirrus which 

 travelled back with the eyes, but which, being a hindrance 

 to sw^imming and burrowing, had become quite rudimentary. 

 Gerstaecker suggests the homology of it with the frontal 

 cirrus (Stirnzapfen) of the Ostracoda. From lack of any 

 detailed study of its finer structure it has, in fact, been 

 very generally claimed as a sensory organ of some kind. 

 A close microscopic study of it, however, shows very clearly 

 that it is an excretory organ. 



Fig. 70 shows the surface view, and Fig. 71 shows it in 

 longitudinal section— both are taken from adult specimens 

 of L. productus. Fig. 38, p. 160, shows it in the Nauplius of 

 Apus cancriformis. From this last figure we may perhaps get 

 a hint as to its real origin and significance, viz. that it was 

 the larval excretory organ. 



First, however, as to its structure. A longitudinal section 

 shows us a number of fine connective-tissue strands stretched 

 between the thin cuticle of the organ and the connective 

 tissue belonging to the longitudinal muscle-bands, which 

 bend round over the mid-gut to be attached close to the 

 prostomium. These fibres lie in the full blood stream issu- 

 ing from the aorta cephalica, and form a net-work to arrest 

 the blood corpuscles.^ This net-work does not, however, 

 stretch right across, but, as the animal always swims on its 

 back, it forms, as in the drawing, a ground-net to catch those 

 particles which sink, and roll along the bottom. Hence, 

 while the ordinary blood corpuscles shoot through the open 

 part as indicated by the arrow, those laden with excretory 

 matter are caught in the net spread across their path as they 

 roll heavily along. The whole action is purely mechanical ; 



^ This interesting use of the connective-tissue fibres is well illustrated 

 in many parts of Apus, particularly around the large reserve or fat cells. 

 It has already been noticed by Grobben in another connection. 



