462 THE LANCELETS— CEPHALOCHORDA 



Chordate type. The nervous system is primitive in several 

 ways, e.g., in the very slight development of the brain region, 

 in the separateness of the dorsal and ventral roots, and in the 

 somewhat embryonic position and character of the nerve-cells. 

 There is also primitiveness in the persistent unsegmented noto- 

 chord, the large number of gill-slits, the persistence of myo- 

 tomes all over the body-wall, the numerous separate nephridia, 

 the numerous segmentally arranged gonads. Compared with 

 fishes, the Lancelets are primitive in their negative characters, 

 namely, in the absence of limbs, skull, jaws, differentiated brain, 

 sympathetic nervous system, cerebral eyes, ears, definite heart, 

 pancreas, spleen, genital ducts. Even the mode of ingestion 

 by the ciliary inwafting of particles is very primitive. All 

 things considered, the most straightforward view seems that 

 which regards this type as an offshoot from a primitive verte- 

 brate stock. It is necessary, however, to consider an objection 

 that is often raised : may not the apparent primitiveness have 

 resulted from degeneration ? 



III. It is always possible to interpret comparatively 

 simple types in two ways in their relation to less simple types 

 belonging to the same series. On the one hand, we may 

 suppose that the simpler types represent lower levels in the 

 evolution of the series in question, or offshoots from these lower 

 levels. Thus, it is usually supposed that Selachian fishes re- 

 present an older stock than Teleostean fishes — a lower branch 

 of the piscine genealogical tree. On the other hand, we may 

 suppose that the simpler types are the results of a process of 

 degeneration from higher types. Thus, it is usually supposed 

 that the typical Ascidians have undergone degeneration. 

 To take an illustration from another field, the winglessness of 

 Collembola and Thysanura is doubtless primitive and so are 

 many other features in these simple insects ; but the wingless- 

 ness of lice or of fleas is doubtless secondary, and the result of 

 degeneration. Our immediate problem, then, is this : How far 

 may the relative simplicity of Amphioxus be regarded as the 

 result of a derivation of this type from some higher Chordate 

 type ? 



When we consider the intricacy of certain structures in 

 Lancelets, such as the oral vestibule, the pharynx, the atrial 

 cavity, and the nephridia, we do not find any warrant for re- 



