THE EVIDENCE OF THE SKELETON 137 
This plastron, it is true, is found in other animals, all of which 
are members of the scorpion tribe, except in one instance, and this, 
strikingly enough, is the crustacean Apus—a strange primitive form, 
which is acknowledged to be the nearest representative of the 
Trilobita still living on the earth. None of these forms, however, 
possess any sign of an internal cartilaginous branchial skeleton, 
such as is possessed by Limulus. Scorpions, Apus, Limulus, are 
all surviving types of the stage of organization which had been 
reached in the animal world when the vertebrate first appeared. 
THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED 
OF SoFT CARTILAGE, 
Searching through the literature of the histology of the cartila- 
ginous tissues in invertebrate animals, to see whether any cartilage 
had been described similar to that seen in the branchial cartilages of 
Ammoceetes, and whether such cartilage, if found, arose in a fibrous 
tissue resembling muco-cartilage, I was speedily rewarded by finding, 
in Ray Lankester’s article on the tropho-skeletal tissues of Limulus, 
a picture of the cartilage of Limulus, which would have passed muster 
for a drawing of the branchial cartilage of Ammoceetes. This clue 
I followed out in the manner described in my former paper in the 
Journal of Anatomy and Physiology, and mapped out the topography 
of this remarkable tissue. 
Limulus, like other water-dwelling arthropods, breathes by means 
of gills attached to its appendages. These gill-bearing appendages 
are confined to the mesosomatic region, as is seen in Fig. 59; and these 
appendages are very different to the ordinary locomotor appendages, 
which are confined to the prosomatic region. Each appendage, as is 
seen in Fig. 58, consists mainly of a broad, basal part, which carries 
the gill-book on its under surface ; the distal parts of the appendage 
have dwindled to mere rudiments and still exist, not for locomotor 
purposes, but because they carry on each segment organs of special 
importance to the animal (see Chapter XI.). As is seen in Fig. 58, 
the basal parts of each pair of appendages form a broad; flattened 
paddle, by means of which the animal is able to swim in a clumsy 
fashion. Very striking and suggestive is the difference between 
these gill-bearing mesosomatic appendages and the non-gill-bearing 
locomotor appendages of the prosoma. 
