THE EVIDENCE OF THE SKELETON 1 37 



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 oe Eespiratory 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 

 Ammoccetes, 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 Ammoccetes. 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 XL). 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 prosoina. 



