SECT. XII RELATION OF APUS TO CRUSTACEA 187 



Abdomen .and Caudal Spine. — These probably re- 

 present nine ^ segments, of which the first five carry 

 leaf-Hke gills, four (the last of which develops into the 

 caudal spine) remaining limbless ; these latter are to 

 be compared with the five or six limbless segments 

 of Apus. The caudal spine is a development of 

 the anal segment homologous with the tail plate of 

 Lepidurus. i.e., of those Apodidae in which the anal 

 segment is produced posteriorly into a flat plate. 



According to its external organisation, therefore, 

 Limulus is an Apus-like animal, especially adapted 

 for living on mud under a shell. The shell is vaulted 

 and the body correspondingly compressed against its 

 roof, so as to allow the limbs, &c., to function. In 

 this way we think that the chief differences between 

 Apus and Limulus can be explained. It is important 

 to bear in mind this general principle on which 

 Limulus has been modified as a key to its special 

 organisation. 



It is worth pointing out that Packard, quoting 

 from Dr. Gissler, describes the method of moulting 

 in Limulus and Apus as being essentially alike. 

 This, however, need be no more than a case of 

 analogy. 



In the following detailed comparison we shall find 

 that in some points Apus is the more specialised, in 

 others Limulus, but there can be little doubt that, of 

 the two, Apus stands nearer to the original Crusta- 

 cean-Annelid. 



■• Pacliard gives this number for the abdomen of Limulus. 



