1895.] Br Gaskell, The Origin of Vertebrates. 43 



how it was then said by an eminent comparative anatomist — These 

 coincidences and resemblances between the structure of man and 

 the ape are merely trivial and have no weight in comparison to 

 the great fact that man has a hippocampus minor in his brain and 

 the ape has none — to which was afterwards added — man has an 

 opponens pollicis and the ape has none. 



I venture to compare the argument about the ccelom with that 

 of the hippocampus minor in the earlier controversy, and just as in 

 the latter it was shown that the ape did possess a hippocampus 

 minor so I propose to show, not only that there is no vital differ- 

 ence between the ccelom of the Vertebrate and that of Limulus or 

 Scorpio, but also that no stronger evidence could be adduced of the 

 correctness of my theory than is afforded by this very appeal to 

 the ccelom. 



It is clear that the ccelomic cavities which must be compared 

 in order to test the truth of my theory are in the case of 

 the Vertebrate the head cavities of Balfour and in the case of 

 Limulus the prosomatic and mesosomatic ccelomic cavities, for the 

 whole strength of my argument is based upon the comparison of 

 the prosomatic and mesosomatic segments of Limulus with the 

 cranial segments of Ammocoetes. 



Fortunately a recent paper by Kishinouyi on the development 

 of Limulus allows such a comparison to be made; he not only 

 shows that the ccelomic cavities in each segment of Limulus are 

 formed by the splitting of the splanchnopleur and somatopleur in 

 the same way as in Vertebrates, but also that although each one 

 of the mesosomatic segments possesses a ccelomic cavity, this is not 

 the case for the prosomatic segments. In these latter the 1st 

 ccelomic cavity is a large prseoral one, common to the segment of 

 the 1st appendage and all the segments in front of it ; the 

 segments belonging to the 2nd, 3rd and 4th appendages have no 

 ccelomic cavities formed in them ; the 2nd ccelomic cavity belongs 

 to the segment of the 5th appendage and then each subsequent 

 segment possesses a ccelomic cavity. Clearly on my theory the 

 1st large praeoral ccelomic cavity common to the most anterior 

 segments of Limulus corresponds to the 1st large prseoral head 

 cavity of Balfour and of v. Wijhe. Balfour's 2nd head cavity is the 

 mandibular associated especially with the Vth nerve. According 

 to my view the motor part of the Vth represents the five locomotor 

 appendages of Limulus, and we see already in Limulus that the 

 segments corresponding to the three foremost of these appendages 

 do not form ccelomic cavities. Posteriorly to the segments belong- 

 ing to the Vth nerve a head cavity is found in each segment in 

 Elasmobranchs and Ammocoetes just as in Limulus, the main 

 difference between v. Wijhe and Balfour being that according to 

 the former Balfour's hyoid segment is in reality double. 



