1895.] 



Dr Gaskell, The Origin of Vertebrates. 



31 



Supra 



oesophageal 



ganglia 



II 



Ammoccetes 

 olfactory organ 



optic segment, lateral 



eyes 

 pineal eyes 



Limulus 



olfactory organ (well 

 seen in Thelypho- 

 nus) 



lateral eyes 



Infra 

 oesophageal 



and 



abdominal 



ganglia 



median eyes 

 comparison not yet worked out 



tentacles 



velar appendage 



locomotor appendages 



last locomotor 

 pendage 

 corresponding epimeral sensory nerves 



ap- 



IX 



X, 



auditory organ 



thyroid 



hyo-branchial 



1st branchial 

 2nd „ 

 3rd 



XJ 7th 



flabellum 

 genital operculum 

 branchial operculum 

 (found in Thely phonus, 

 &c, not in Limulus) 

 1st branchial 

 f2nd „ 

 3rd „ 



5th 



The comparison of the sensory part of the Vth with the epi- 

 meral nerves of Limulus, the meaning of the Illrd and IVth nerves, 

 of the sensory branches of the vagus group &c, must be left until 

 the completion of Miss Alcock's investigation of the course and 

 distribution of all the branches of the cranial nerves in Ammoccetes, 

 and their comparison with the homologous nerves of Limulus and 

 Scorpio. 



Two points of interest may be mentioned here, viz. the genesis 

 of the medulla oblongata and the reason for the invariable position 

 of the glossopharyngeal as the separate nerve of the first true 

 branchial segment. 



The comparison between the medulla oblongata of Ammoccetes 

 and of Limulus shows that in the latter, the only nerve belonging 

 to the lamellar appendages which arises from the medulla ob- 

 longata is the nerve of the operculum, i.e. the Vllth nerve of 

 Ammoccetes, the IXth and Xth nerves arising from the separate 

 abdominal ganglia; in other words, only one ganglion and nerve 

 belonging to the abdominal or mesosomatic appendages has 



