102 



5. On the Development of the Lung-Books in Scorpio fulvipes. 



By Malcolm Laurie, B.Sc, F.L.S. 



eingeg. 29. December 1891. 



The following observations on the development of the lung-books 

 oi Scorpio fulvipes , and the light thrown by them on the probable 

 mode of origin of these structures from the gills of an aquatic ances- 

 tor, were made by me while investigating the embryology of this form, 

 an account of which was published in the Quarterly Journal of Micro- 

 scopical Science, Vol. XXXII, p. 587. In the earlier stages the lung- 

 books present no special points of interest being simple ingrowths of 

 the ventral surface of the abdominal segments precisely similar to 

 what I have described in Euscorpius italiens i. In the latest stage 

 which I have examined, however, the lamellae are well developed and 

 have the same structure as in the adult though they are not so nume- 

 rous. They differ from those of the adult however , very strikingly in 

 position. In the adult the lamellae stand perpendicular to the ventral 

 surface of the body and directly over the stigmatic opening (fig. 1). 



Fig. 1 . Diagrammatic transverse section of adult Scorpio to shew the position 

 of the lamellae of the lung-books. 



Fig. 2. Diagrammatic longitudinal section of Embryo Scorpio, a anterior; 

 p posterior end. 



The lamellae are further placed parallel to the axis of the body, i. e. 

 with their flat surfaces directed towards the sides of the body and their 

 edges towards the anterior and posterior ends. In the embryo, on the 

 other hand , the lamellae lie horizontally and parallel to the ventral 

 surface (fig. 2) resembling in this respect the lamellae of the Aranei- 

 dae. The stigmatic opening is situated at the postero-internal angle 

 of the lamellae. The change in position of the lamellae in passing 

 from the embryonic to the adult condition thus involves a movement 

 of 90° from the horizontal to the vertical position and a further twist 

 of about 45° to bring the lamellae parallel to the axis of the body. 



1 Q. J. M. S. Vol. XXXI. 



