LIMULUS AN ARACHNID. 545 
inserted into the soft ventral integument near the base of 
each double gill-plate. These muscles serve (together with 
others that enter the appendage itself) by their contractions 
to move the gill-plates in the water and so aid in aquatic 
respiration. The position of the insertion of each muscular 
mass is marked by a deep funnel-like depression of the 
integument. From the external surface this depression 
appears as a “stigma,” which we have already described as 
the parabranchial stigma. The funnel-like depression has a 
narrow mouth which is often as much as half an inch in 
length. Internally the invaginated cuticle stands up as a 
flexible tendon clothed with fibrous tissue and giving attach- 
ment to the muscle already mentioned. 
In Limulus we find a pair of these “ muscle-stigmata,” 
right and left behind the genital operculum, and a pair 
(right and left) behind each of the lamelliform fused appen- 
dages which carry the gill-books. 
We have only to suppose the appendages carrying the 
gill-books not to have fused as yet in the middle line, and 
the muscular stigmata to have become greatly developed 
(perhaps by increased development of the muscle aiding in 
aquatic respiration when the appendage itself grew small 
and therefore less efficient) and we have at once the gill-book 
sinking within the area of the stigmatic pit, Pl. XXIX, 
fig. 25. 
A very important feature in the supposed further develop- 
ment is the correspondence of the atrophy of the muscle 
(which atrophy is required to fit in with our hypothesis, and 
to convert the muscle-pit into a pulmonary sac) with the 
changes in the structures which would necessarily result 
were the physiological conditions gradually to become such 
as to favour aérial in place of aquatic respiration. The 
violent agitation of the gills by the muscle attached to the 
stigmatic pit would become useless, supposing an exposure 
of the gill-lamelle to the atmosphere became by degrees 
habitual with the ancestral Arachnidan. In proportion as 
these hypothetical creatures acquired the habit of aérial 
repiration—the deepening and arching in of the stigmatic 
pit would be favoured, and the atrophy and final disappear- 
ance of the muscle which was attached to its inner surface, 
and mechanically brought it into existence, would also be 
directly promoted. 
A further confirmation of the view now advanced is found 
in the remarkable East Indian Arachnid Thelyphonus. This 
Arachnid has not four pairs of lung-sacs like Scorpio, but 
only two pairs, corresponding to the two foremost lungs of 
