1 8 MORPHOLOGICAL TYPES 



at the stage in question (in this case Amphioxus). A primitive 

 character of this kind is shown by the vascular system of 

 Amphioxus. There are also negative characters, for the later 

 evolutionary stage may possess structures which the present 

 stage lacks. The absence of a specialised head in Amphioxus 

 is an example of a primitive negative character of this kind. 

 Then there are characters of which it cannot be said that they 

 are simpler than those of the next evolutionary stage, nor that 

 they lead on to them, but which can be considered as his- 

 torically primitive in the sense that they occur at early stages 

 but not at later ones. The ciliary method of feeding is an 

 historically primitive character of this kind : it preceded 

 the jaw-method of feeding in time, but was not simpler than 

 the latter method, nor did it lead up to it. 



All primitive characters imply the possibility of progress 

 in evolution. On the other hand, there are certain characters 

 which have not only not contributed to the progress in evolu- 

 tion to the next stage, but have debarred their possessors 

 from ever evolving to that stage. Such specialised or 

 secondary characters are typified by the atrium of Amphioxus. 



The analysis may conveniently be set out in tabular form : 



Amphioxus 



Primitive Characters. 



Ciliary mode of feeding, with endostyle ; 



Epidermis one-cell thick ; 



Afferent nerve-fibres derived from sensory cells ; 



Complete row of segmented myotomes from front to 



rear ; 

 Very slight specialisation of brain ; 

 No specialised head ; 

 No paired limbs or paired sense-organs ; 

 No specialised heart ; 

 Gonads segmental, without special ducts ; 

 Nephridia ; segmentally arranged ; 

 Simple and unbranched liver diverticulum. 



