104 



the young of Scorpio would result, must have borne a series of gill 

 lamellae on its anterior margin or surface — a type of appendage quite 

 unknown among either recent or fossil aquatic Arachnids, and which 

 would place the gills in a very exposed position. 



The theory propounded by Macleod^ and suggested to him by 

 a study of the lung-books of Spiders, is briefly, as follows: He 

 supposes a pair of platelike abdominal appendages united, like those 

 of Limulus, in the middle line and bearing branchial lamellae on their 

 posterior surface, to have become gradually fused round the margin 

 to the ventral surface of the body. The process might perhaps be 

 better described as an extension of the line of attachment along the 

 sides of the appendage so as to convert the space between the appen- 

 dage and the body wall into a chamber opening to the exterior along 

 a greater or less extent of its posterior border. This theory has much 

 to be said for it. In the first place, it avoids the difficulty as to 

 the original position of the lamellae which we met with in consi- 

 dering Prof. Lankester's theory, as the air space would occupy 

 the position not of the axis but of the free end of the lamellae. Ano- 

 ther strong point in its favour is that it is easy to see how the 

 change might arise step by step as the animals took more and more to 

 a terrestrial mode of existence ; every step being of advantage as tend- 

 ing to keep the gills longer in a moist condition. The change would 

 thus be a gradual one instead of being sudden as in Professor Lan- 

 kester's hypothesis. 



There is only one point of importance in which I would differ 

 from Made od. He would derive the lung-books from a pair of 

 appendages fused in the middle line and adduces in favour of this the 

 presence of a transverse groove connecting the lung -books in many 

 forms. The Araneidae are, however, a highly specialized group and 

 the absence of any such structure in the lower Arachnids is to me a 

 strong argument against its having any phylogenetic significance. In 

 the embryo Scorpio the lung-books shew no trace of any structure 

 uniting them , but are separated by a broad , shallow , longitudinal 

 groove which runs down the whole length of the abdomen. 



The form, from which I would suggest that the lung-books of the 

 Arachnids have been derived, is one which I have described as exist- 

 ing in Slimonia — one of the fossil Eurypterids from the Upper Silu- 

 rian rocks. In Slimonia the abdominal appendages consist of a pair of 

 separate plates on each segment each of which bears on its posterior 

 surface branchial lamellae. These lamellae are attached towards the 



3 Arch. Biol. Vol. V. 



