2o6 THE APODID^ part ii 



of view. This specialisation of the blood vascular 

 system in Limulus is a very good illustration of the 

 principles stated on p. 117. The compression of the 

 body of Limulus against the vault of its own shell 

 would lead to the development of special vessels to 

 supply those parts which, because of compression, 

 would not otherwise receive their proper share of 

 blood. Thus we may consider the circulatory vascular 

 system of Limulus either as a modification of that of 

 the original Crustacean-Annelid, or as secondarily 

 acquired. The latter view is more probably the 

 correct one. In the first place, the type of the system 

 is hardly that of an Annelid, and in the second place, 

 the arterial envelopes surrounding the nerves are 

 clearly secondary specialisations in adaptation to the 

 peculiar physiological needs of the animal. 



The Annelidan character of the long dorsal vessel 

 with eight pairs of ostia needs no special notice ; it 

 speaks for itself in showing that at least in this respect 

 Limulus is not so far removed from the Annelids as 

 its highly specialised form would have led us at first 

 sight to imagine. 



The genital organs in Limulus are considerably 

 more specialised than in Apus. The comparative 

 shortness and flatness of the body hinders the primi- 

 tive metameric arrangement which we find in the latter 

 animal. The eggs appear to develop towards the 

 lumen of the gland instead of outwards towards the 

 body cavity. This advance on Apus is what we should 

 expect from the compression of the whole body, and 

 the consequent diminution of the body cavity. 



