ON THE ORIGIN OF VEETEBRATES FROM ARACHNIDS. 365 



the (Jevelopment of the ova and spermatozoa, all supply us 

 with abundant evidence of the great morphological and histo- 

 logical specialisation of the Arachnids, and their structural 

 similarity to Vertebrates. 



There is another point which deserves more than passing 

 notice. In insects the sexual organs are developed (Acilius) 

 from the wall of one, or at most two, mesoblastic somites, and 

 are carried by the growth of these organs to the hsemal surface; 

 the adult organs are never reticulated. In Scorpio (and 

 Liraulus ?) it is just the reverse ; the sexual organs arise as a 

 median longitudinal band of cells underlying the nervous 

 system, and extending the whole length of the primitive 

 abdomen — that is, over at least seven segments ; moreover, in 

 the adult the sexual organs are remarkable for their reticula- 

 tion. In the adult Scorpion the sexual gland is neural in 

 position, and is composed of three longitudinal tubes united 

 by transverse ones. Now in Protopterus the testis has exactly 

 the same arrangement of longitudinal and transverse bars as 

 in Scorpio, the only difference being the position of the outlets 

 and the small size of the median longitudinal tube. In other 

 words, it is a most significant fact, when viewed in connection 

 with all that has preceded, that the sexual organs of Scorpio 

 and Limulus, in their exceptional position, structure, and 

 development, should resemble in these very features the sexual 

 organs of primitive Vertebrates. 



XIV. Embryology. — Kleinenberg's admirable observations 

 on the development of Lopadorhynchus afford the first secure 

 foundation for the interpretation of the embryology of the 

 higher segmented animals. They teach us not only exactly 

 what the gastrula of a segmented animal is, but also what it is 

 not. As long as such forms show no trace of concrescence, of 

 ccelomic diverticula, or of any connection between an un- 

 doubted gastrula and an undoubted " primitive streak," we 

 must, in order to explain the facts of Arthropod and Verte- 

 brate embryology, follow other paths than those laid down by 

 the concrescence theory, the coelom theory, or any other theory 



VOL. XXXI, PART in, — NEW 8ER. B B 



