326 WILLIAM PATTEN. 



plan, the only difference beiug that in the other segments the 

 coxal sense-organs and consequently the coxal nerve are 

 smaller, and there seems to be no lateral ganglion in the 

 adult. 



Two pairs of haemal nerves arise from each of the six 

 thoracic neuromeres ; they are small and light coloured, and are 

 probably entirely motor, supplying the innermost muscles, and 

 probably some of the anterior viscera (Figs. 1 and 3, a. h. n. 

 andj5. h. n.). 



Segmental Sense-organs and Ganglia. — One of the 

 most important evidences of the Annelid origin of Vertebrates 

 has been the similarity between the segmental sense-organs of 

 fishes and Annelids. The value of this evidence has recently 

 been destroyed, because it is now known, from the researches 

 of Beard and Allis, that the lateral line-organs of fishes are 

 formed by a backward growth of cranial sense-organs, and 

 that their segmental arrangement is only secondarily acquired. 

 Moreover, Beard's researches show such an unsuspected com- 

 plication of cranial ganglia, sense-organs, and nerves, that it is 

 difficult, if not impossible, to compare them with similar 

 parts in the body. To say the least, his observations do not 

 strengthen the Annelid theory, because the latter cannot 

 explain this extraordinary difference between the cranial and 

 spinal nerves, its aim and only hope being the reduction of the 

 ancestral Vertebrate to a collection of like, not unlike m eta- 

 meres. We certainly do not have this difficulty with the 

 Arachnid theory, because the distribution and history of the 

 thoracic sense-organs, ganglia, and nerves of Scorpio and 

 Limulus, resemble in a striking way those of the corresponding 

 parts of Vertebrates. For example, in Scorpio (1) the pedal 

 nerve, its neural and lateral ganglia, and its purely sensory 

 branch, or coxal nerve, the coxal and the segmental sense-organs, 

 and the anterior and posterior haemal nerves, — all these features 

 produce in each thoracic neuromere a complex condition 

 similar to that found in a typical cranial neuromere of Verte- 

 brates. (2) Omitting the fore and accessory brain, and using 

 the cranial ganglia as guides, there are in the head of 



