No. 2.] THE EMBRYOLOGY OF LIMULUS. 227 



in, behind it, has begun. In L we have the modifications of /, 

 which result in the formation of the gill-book. On the pos- 

 terior face of the appendage the gill-leaves are budding out. 

 In A we have the Arachnid modifications of /. On the right 

 side the post appendicular insinking has resulted in the forma- 

 tion of the pulmonary sac from the anterior wall of which, — and 

 which is plainly the posterior surface of the appendage — the 

 pulmonary leaves are being produced (cf. Laurie, '90, pi. XVII, 

 Fig. 47). At the left side of the figure the same conditions 

 are carried further, and the opening of the pulmonary sac is 

 now reduced to the narrow spiracle. In all the figures the 

 anterior surface of the appendage is crossed-lined, the pos- 

 terior is black, the invaginated portion of the ventral surface 

 dotted, the rest of the ventral surface is white. The arrow 

 points towards the 'anterior end of the animal. 



When the comparison is made in this way the similarities, as 

 to appendages and lamellae, are seen to be very close. When 

 viewed from the histological standpoint the resemblances are 

 so exact that the description of the pulmonary organ of the 

 spider or of the scorpion will apply almost, word for word, as 

 shown above, to the gill book of Limulus. This, taken in con- 

 nection with the fact that the very appendages which in the 

 scorpion (IX-XII) are converted into the lung-books, are in 

 the Limulus the bearers of gill-books, and that appendages 

 VIII of the scorpion (the pectines) have a structure also easily 

 reducible to the gill-book of the corresponding somite of the 

 horse-shoe crab, place the homologies of the organs in such a 

 light that few identities of structure and of phylogeny are more 

 certain. 



The consideration of the relations existing between the lungs 

 and the tracheae of the Arachnids will be taken up later. 



The Relationships of Limulus. 



It would seem hardly necessary to review in detail the 

 discussion of the systematic position of Limulus since it has 

 been done by almost every author who has treated of its 

 anatomy or ontogeny or who has studied the spiders. Yet 

 some space must be devoted to it because of the new facts 



