40 pROCEr-mNGs of thk 



judgment. All are liable to it and it may be that something of 

 till' kind is here at work. Though he could not follow Dr. Graskell 

 in the theory put forward by him as to the origin of Vertebrates, he 

 recognised very gratefully the value of the observations on many 

 details of structure to which it had led that distinguished physio- 

 logist, and also the new observations which it had called forth on 

 tlie ])art of other naturalists, such as the interesting additions to 

 our knowledge of tlie head-shield and the body-scales of Gcplial- 

 a^pis which had just been placed before the meeting by Dr. Smith 

 Woodward. Jle thought the Society was to be congratulated on 

 n very interesting debate. (In the further course of the discussion 

 Sir Kay Lankester stated that whilst he considered Amphioxus 

 and the Ascidian tadpole to present in many points of structure 

 a very much more primitive phase of the Vertebrate group than 

 do either Lampreys or Pishes, he held that they were also specially 

 modified and degenerate each in its own way, and were not closely 

 representative of tbe main line of descent, lie considered that 

 the remains of the earliest known fossil fishes, on account of their 

 necessardy incomplete condition. Avere not capable of throwing 

 much light on the question of Vertebrate ancestry. He was led 

 to the conclusion that Balanoglosms threw some light on the 

 subject, and he drew attention to the remarkably complex brain 

 and cerebral respiratory pits of the Nemertine worms and the 

 dorsal median as well as lateral nerve-cords of those creatures, 

 which had led Hubrecht long ago to suggest their close connection 

 with the remote ancestors of A^ertebrates. A large survey of the 

 facts of animal structure, even including that of unfamiliar marine 

 worms, was necessary in order to form a reasonable judgment on 

 the question of Vertebrate ancestry.) 



Dr. P. Chalmers Mitchell, F.E.S., T.L.S., remarked that con- 

 sideration of the general morphology of the nervous system enables 

 us to place the Vertebrates in their true perspective amongst 

 the various Invertebrate groups. In the Ca?lentera, as shown by 

 the Hertwigs, the nervous system frequently appears as a diffuse 

 layer of cells and fibres underlying, and in close connection 

 with, the epidermis, whilst there is much evidence that a 

 similar primitive condition underlies the various presentations 

 of the nervous system in higher groups. Even amongst the 

 Ccelentera, two processes coincideutly or independently result 

 in modification of the primitive simplicity. The original diffuse 

 layer may become thickened in definite regions, forming, for 

 instance, rings round apertures or radial bands, whilst in tlie 

 intervening areas it may be obliterated. The thickened bands or 

 rings may migrate inwards and lose their intimate connection 

 with the epidermis. Similar processes varying in position and 

 extent of their incidence have led to many different arrangements 

 of tlie nervous system in the higher groups. 



In tlu! Turbellaria, inward migration has taken place, and two 

 ventro-lateral cords have been formed. 



