28 METAMORPHOSES OF MAN 



shortly after the nervous system. The digestive tube 

 is seen at a later period. 



This order of succession seems necessary, when we 

 consider that the embryo receives its nutritive supply 

 from external sources, through the intervention of the 

 blood-vessels. When this necessity does not exist, 

 the order of phenomena is in all probability altered ; 

 in fact, we see this alteration take place in most of the 

 invertebrates whose development we are acquainted 

 with. The digestive system is formed before the cir- 

 culatory organs, and occasionally the latter are com- 

 pletely absent, even after the young animal has been 

 hatched, and has begun life on its own account. 



We should be almost inclined to associate this law 

 with the imperfect development of the circulatory 

 organs of certain adult invertebrates, were it not that 

 the same thing has been shown to occur in animals 

 whose circulation is complete and closed. If, then, we 

 would understand why this system is so slowly deve- 

 loped, we must examine another phenomenon, which 

 though confusing to some, will be easily appreciated 

 by those who can form a proper estimate of the function 

 of the general cavity of the body and the office of its 

 contained liquid.* In those animals which have a 

 free general cavity, the various organs of the body are 



* Elsewhere I have described at some length, the function of 

 this general cavity (Souvenirs d'un Naturaliste) ; my remarks may 

 thus be briefly summed up : — " Invertebrates have as a general 

 rule, neither lymphatic nor chyliferous vessels. Moreover, 

 they have no connective tissue in the literal sense of the word. 

 Consequently we find the various organs separated from each other 

 by spaces or lacunce, filled with a peculiar liquid which represents 

 both lymph and chyle. Besides, when the circulation is incom- 

 plete, the blood is poured from the vessels into these lacunse and 



