3 66 



THE ORIGIN OF VERTEBRATES 



CA.t 



not, however, attempted to examine the whole sense-organ for the 

 purpose of estimating their number and arrangement. 



As is seen in Fig. 149, they possess a fine tubule of the same 

 character as that of the neighbouring sense-organs, which apparently 

 terminates at the apex of the projecting spike. They appear to 

 belong to the same group as the other poriferous sense-organs, and 

 are of special interest, because in their appearance they form a link 

 between the latter and the poriferous sense-organs which charac- 

 terize the pecten of the scorpion (cf. Fig. 152, C). 



Such, then, is the structure of this remarkable sense-organ of the 

 flabellum, as far as I have been able to work it out with the materials 



at my disposal. It is 

 evident that the flabellar 

 organs, apart from the 

 spike-organs, are of the 

 same kind as those de- 

 scribed by Patten on the 

 mandibles and chelre of 

 Limulus, and therefore it 

 is most probable that the 

 nerve - terminals in the 

 chitinous tubules, and 

 the origin of the latter, 

 are similar in the two sets 

 of organs. 



These organs, as Patten 

 has described them, are 

 situated in lines on the 

 spines of the mandibles of the prosomatic locomotor appendages, 

 and are grouped closely together to form a compact sense-organ 

 on the surface of the inner mandible (Lankester's epicoxite) (i.m. 

 in Fig. 155), so that a surface- view of the organ here gives 

 the characteristic appearance of these poriferous sense-patches. 

 Precisely similar organs are found on the chilaria, which are, in 

 function at all events, simply isolated mandibles, to use Patten's 

 terminology. 



On the digging appendage (ectognath), as the comparison of 

 Fig. 155, A and C, shows, the mandibular spines are almost non- 

 existent, and the inner mandible or epicoxite is not present, so that 



Fig. 149.— Spike-Organ op Flabellum. 

 ch.t., chitinous tubule. 



