610 PROFESSOR E. RAY LANKESTER. 
early Scorpion ancestors, there was blood. A blood space has 
become converted into an air space just as an air space (that 
of the investing sac) has become converted into a blood space. 
That a blood space should have become converted into an 
air space is not exceptional. All trachez in Arthropoda are 
potenttal blood-vessels, and their blood-vessels are potential 
trachee. The air-carrying spaces of Arthropoda have been 
in fact in many cases probably produced by a direct conver- 
sion of blood-vessels. 
The changed contents of the Scorpion’s as compared with 
the King Crab’s respiratory appendage does not affect the 
morphological significance of its parts nor the importance to 
be attached to the evidences of its having once projected on 
a free surface, although now sunk within asac formed by a 
recess of the body surface. 
The minute embryological history of the Scorpion’s lung 
book is the evidence which we now want in order to actually 
demonstrate that the primitive stigma is the orifice of in- 
vagination of the investing sac into which the lamelligerous 
appendage sinks, and that the opening into the axis of the 
appendage from the surface is a secondary formation, pro- 
duced after the primitive stigma had been occluded and 
completely closed by the adhesion to the lips of that orifice 
of the axis of the in-sunken lamelligerous appendage. 
So much as is already known of the embryological history 
of the Scorpion’s lamelligerous lung sacs is not opposed to 
the view here advocated. Rudimentary appendages, which 
in the embryo project from the surface of the segments in 
which the pulmonary organs are subsequently found, dis- 
appear from view at the same time as certain pits are formed 
in their immediate vicinity. These pits and their orifices of 
invagination are, according to my view, not the air-contain- 
ing chamber and its permanent external opening, but the 
investing sacs (the homologues of the hollow parabranchial 
tendons of Limulus) in which the lamelligerous appendage 
is enclosed, and which cease after their formation to com-~ 
municate with the exterior. 
§c. ALIMENTARY CANAL.—Though there are very con- 
siderable differences between the alimentary canal and its off- 
growths in Limulus and in the Scorpion, yet there are some 
remarkable agreements of a fundamental character. The differ- 
ences, such as they are, can be viewed as the results of special 
adaptation. There is the same difficulty with regard to the 
facts relative to the Scorpion’s alimentary system as in re- 
gard to all its viscera. I shall rely upon Newport, but I 
