McCrady.] 190 [Dec. 3, 



dages. The great difficulty is presented by Sagitta: but here we 

 have a Nematode with very extraordinary conditions of parasitism, 

 which make it the analogue among Radiata of the Pteropoda among 

 Mollusca. An extraordinary form therefore we may naturally expect, 

 and find it no bar to the assignment of Sagitta to the same branch of 

 the Animal Kingdom with which its nearest allies connect them- 

 selves. 



On the other hand, the Annulata proper are undoubtedly, as 

 Cuvier taught, members of the great Articulate series, having the 

 distinctly arthro-cylindrical body, which is archetypal in that series. 

 The same remark applies to the Rotatoria, which hold a position in 

 respect of the Crustacea and Arachnida, analogous to that held by 

 the Annulata proper to the Myriapoda and Insecta. The Leeches at 

 the base of the Articulate series seem to be analogues of the Lam- 

 preys at the base of Vertebrata, and to have similar conditions of 

 parasitism. 



The approximation of the Gephyrians to the Echinoderms I have 

 treated as a matter of course ; their embryology is the same, but they 

 differ in the parasitism of their adult condition. I include among 

 them Balanoglossus and Phoronis, the latter being a polypoid form 

 allied to the Sipunculidse. The extraordinary variety of form and 

 structure among Gephyrians, affords an excellent field for the study 

 of the modifying effects of extraordinary conditions of parasitism. 



The progress of real knowledge is not usually cataclysmal. A 

 vague striving after the subversion of everything done by our pre- 

 decessors, is as unscientific as an unreasoning acquiescence in their 

 results. Cuvier showed good ground for the belief that there are 

 four great groups presented to us in the Animal Kingdom. Our im- 

 proved acquaintance with structure renders it not only necessary for 

 us to adopt a fifth group, the Protozoa, either as a provisional or as a 

 permanent arrangement, but tends constantly to convince us that 

 Cuvier's four groups are not all equally disparted, the one from the 

 other. It seems to me to prove that there is an affinity (which may 

 lead to the establishment of an order of homologies) between the 

 Radiata and the Articulata on the one hand, and between the 

 Mollusca and the Vertebrata on the other; while at the same time 

 that it approximates these groups two and two, it tends to widen 

 proportionately the distance between the Radiata- Articulata series on 

 the one hand, and the Mollusca- Vertebrata series on the other. Yet 



