20 THE APODID^: PART I 



segments anterior to it which are rudimentary, and 

 which disappear in the development of the higher 

 Crustacea. 



The anal segment is provided with two long 

 cercopoda or cirri, projecting posteriorly and slightly 

 ventrally, and two rudiments, probably of similar 

 appendages, on the posterior dorsal surface of the 

 segment. These four together correspond with the 

 four anal cirri found in some carnivorous Annelids 

 (cf. pp. 85 and 274). The two cirri are stiffened for the 

 greater part of their length by a thickened cuticle 

 covered with setae, and showing slight rings of thinner 

 skin. The tips of the cirri are quite thin-skinned, and 

 seem to function as tactile papillae. 



The posterior dorsal surface of the anal segment 

 is sometimes prolonged into a variously shaped caudal 

 plate or lamella, 1 which we shall find to be the 

 homologue of the caudal spine of the Xiphosuridae. 



THE CUTICLE AND EXOSKELETON. 



The generally thin and flexible Annelidan cuticle of 

 the Apodidae shows local thickenings which may 

 well be recognised as the commencement of the 

 Crustacean exoskeleton. A closer study of these 

 reveals to us the principles of the original formation 



1 Apodidrc having this characteristic have been classed by Leach as a 

 separate genus, Lefidiirns. But Dr. Alfred Walter, to whose memory 

 this essay is dedicated, discovered a form in a desert well in Trans- 

 caspian Russia, A pus Hackelii (Walter), which makes it doubtful whether 

 this division can be sustained. {Bulletin de la Societe Imperiale des 

 Naturalistes de Moscou, 1887.) 



