LINNEAN SOCIETY OF LONDON. 25 



in the head. The formation of a distinct head-region with a large 

 diiferentiated brain, a skull, and cranial nerves, is one of the most 

 important and characteristic features of the structure of the 

 Craniata (Cyclostomes and Gnathostomes). It takes place by 

 the gradual modification of more and more of the segments at the 

 anterior region of the body where are situated the mouth, gill- 

 slits, and paired organs of sense. But this process of cephali- 

 zation has gone much further in the Grnathostomes, where the 9th 

 and 10th cranial nerves become included in the skull, and the 

 corresponding muscle segments are suppressed, than in the 

 Cyclostomes, where these nerves emerge behind tlie rudimentary 

 skull and the muscle segments still in the adult form an un- 

 interrupted series from in front of the mouth to the tip of the 

 tail. Moreover in the Cyclostomes there are no paired limbs, no 

 true teeth, in fact no trace whatever of dermal skeleton, and the 

 testis has not yet acquired any direct connection with the kidney 

 tubules. 



The next point to be studied is the structure of the common 

 ancestor of the Cephalochorda and the Craniata. Now, although 

 Amphioxus is doubtless in some respects a very specialized 

 animal — as for instance in the possession of an atrial cavity — yet 

 it preserves many primitive characters. Judging from its struc- 

 ture, we must conclude that the ancestral Vertebrate was still 

 more uniformly segmented than the primitive Craniate, The 

 head-region was scarcely differentiated at all, there was no skull 

 (probably no cartilaginous axial skeleton at all), a quite rudi- 

 mentary brain, no specialized cranial nerves, no eephalization due 

 to the presence of large paired organs of sense. It is possible 

 that Amphioxus is somewhat degenerate ; but it cannot seriously 

 be urged that it once possessed in well-developed condition those 

 paired sense-organs which have so pi'ofoundly modified the 

 structure of the head-region in the Craniata. For it would 

 be ridiculous to suppose that the modified segments could be 

 I'estored to their original condition of uniformity with the trunk 

 segments ; no trace of the disturbance appearing in either adult 

 or embryo. 



Further, in Amphioxus, there is no dermal or epidermal armour, 

 and primitiveness is shown in the structure of the endostyle, 

 which becomes modified into the thyroid gland in higher forms. 

 Lastly the presence of true uephridia, a type of excretory organ 

 which has been lost in other Vertebrates, links Amphioxus to the 

 lower Invertebrate Coelomata. 



Thus can be traced an irreversible series of stages in the differ- 

 entiation of Vertebrate structure, at the bottom of which we find 

 a much simpler, but still essentially Vertebrate ancestor, probably 

 already extinct in Silurian times. 



Amoug the various Classes of modern Invertebrates we do not, 

 and indeed cannot expect to find any close allies. But the some- 

 what distantly related Enteropneusta (Balanoglossus) seem to 



