Chap. XIL EVOLUTION OF THE RHIZOCEPHALA. 1-35 



gaps would still have to be filled up by means of more 

 or less bold assumptions. Those who have not wan- 

 dered much in this reoion of research would then 

 readily believe that they were standing upon firm 

 ground, where mere fancy had thrown an airy bridge ; 

 those acquainted with the subject, on the other hand, 

 would soon find out these weak points in the structure, 

 but would then be easily led to regard even what was 

 founded upon well considered facts, as merely floating 

 in the air. To obviate these misconceptions of its true 

 contents from either side, it would be necessary to ac- 

 company such a picture throughout with lengthy, dry 

 explanations. This has deterred me from further filling 

 in the outline which I had already sketched. 



I will only give, as an example, the probable history 

 of the production of a single group of Crustacea, and 

 indeed of the most abnormal of all, the Rhizocephala, 

 which in the sexually mature state differ so enormously 

 even from their nearest allies, the Cirripedia, and from 

 their peculiar mode of nourishment stand quite alone 

 in the entire animal kingdom. 



I must preface this with a few words upon the homo- 

 logy of the roots of the Ehizocephala, i.e. the tubules 

 which penetrate from its point of adhesion into the body 

 of the host, ramify amongst the viscera of the latter, and 

 terminate in caecal branchlets. In the pupae of the 

 Ehizocephala (fig. 58) the foremost limbs (" prehensile 

 antennae ") bear, on each of the two terminal joints, a 

 tongue-like, thin-skiuned appendage, in which we may 

 generally observe a few small strongly refractive gra- 



