956 REPORT—1896. 
already mentioned, but also similarity of structure ; thus we see that the blood in 
the gill lamellee and velar appendages of Ammoccetes does not circulate in distinct 
capillaries, but, as in the arthropod appendages, in lacunar spaces, which by the 
fubdivizion of the surface of the appendage to form gill lamellee become narrow 
channels; that also certain of the branchial muscles and of the muscles of the velar 
appendages are of the invertebrate type of so-called tubular muscles. These inver- 
tebrate muscles are not found in higher vertebrates, but only in Ammoccetes, and 
moreover disappear entirely at transformation. 
Origin of the Vertebrate Cartilaginous Skeleton. 
Perhaps, however, the most startling evidence in favour of the homology 
between the branchial segments of Ammoccetes and the branchial appendages of 
Limulus is found in the fact that a cartilaginous bar external to the branchie 
exists in each one of the branchial appendages of Limulus, to which some of the 
branchial muscles are attached in precisely the same way as in Ammoceetes. The 
branchial cartilages of Limulus (see fig. 4) spring from the entapophyses and form 
strong cartilaginous bars which are extra-branchial in position, just as in Ammo- 
coetes, in addition to each branchial bar, a cartilaginous ligament passes from one 
entapophysis to another, so as to form a longitudinal or entapophysial ligament, 
more or less cartilaginous, which extends on each side along the length of the 
mesosoma. In precisely the same way the branchial bars of Ammoccetes are 
joined together along each side of the notochord by a ligamentous band of more 
or less continuous cartilaginous tissue, forming a subchordal or parachordal carti- 
laginous ligament. 
Further, we see that this cartilage of Limulus is of a very striking structure, 
quite different from that of vertebrate cartilage, and that it is formed in a fibro- 
massive tissue which, like the matrix of the cartilage, gives a deep purple stain 
with thionin, thus showing the presence of some form of chondro-mucoid. This 
fibro-massive tissue is closely connected with the chitinogenous cells of the entapo- 
hyses. 
arene is it to find that the branchial cartilages of Ammoccetes possess 
identically the same structure as the cartilages of Limulus; that the branchial 
cartilages are formed in a fibro-massive tissue which, like the matrix of the cartilage, 
gives a deep purple stain with thionin, and that this fibro-massive tissue, to which 
Schneider! gives the name of muco-cartilage, or Vorknorpel, entirely disappears at 
transformation. 
Further, according to Shipley,” the cartilaginous skeleton of the Ammoccetes 
when first formed consists simply of a series of straight branchial bars, springing 
from a series of cartilaginous pieces arranged bilaterally along the notochord. 
The formation of the trabecule, of the auditory capsules, of the crossbars to 
form the branchial basket-work, all occur subsequently, so that exactly those parts 
which alone exist in Limulus are those parts which alone exist at an early stage in 
Ammoceetes. Another distinction is manifest between these branchial cartilages 
and those of the trabecule and auditory capsules, in that the latter do not stain in 
the same manner; whereas the matrix of the branchial cartilages stains red with 
picro-carmine, that of the trabeculze and auditory capsules stains deep yellow, so 
that the junction between the trabecule and the first branchial bar is well marked 
by the transition from the one to the other kind of staining. The difference cor- 
responds to Parker’s * soft and hard cartilage. 
The new cartilages which are formed at transformation, either in places where 
muco-cartilage exists before or by the invasion of the fibrous tissue of the brain- 
case by chondroblasts, are all of the hard cartilage variety. 
The phylogenetic, anatomical, and ontogenetic history of the formation of the 
1 Schneider, Betirdge z. Anat. u. Entnicklungsgesch. der Wirbelthiere. Berliv. 
1879. 
2 Shipley, Quart. Journ. of Micr. Sci. 1887. 
8 Parker, Phil. Trans. Roy. Soc. 1883. 
