546 PROFESSOR E, RAY LANKESTER, 
Scorpio, and to the second and third gill-book-pairs of 
Limulus (woodcut, figs. 12, 13, 14). Nevertheless, as we 
have seen in a previous section of this Essay, the four seg- 
ments of the abdomen posterior to these are each marked by 
a par of shallow stigmata placed in line with the orifices of 
the pulmonarg sacs of the two anterior segments, msg. When 
the internal structure corresponding to these parts is exa- 
mined, it is found that a large muscle (similar to the simi- 
larly placed muscle of Limulus) is inserted into each of the 
four right and four left stigmata in the segments posterior 
to the pulmonary sacs (woodcut fig. 14, m.) The two seg- 
ments into which the two pairs of pulmonary sacs are sunk, 
have xo such muscles. The pulmonary sacs are, therefore, 
to all appearance, enlarged muscular stigmata, from which 
their former muscles have disappeared by disuse and 
atrophy. 
VI. The Entosternite—Leaving now the comparison of 
segments and appendages, which is undoubtedly the most 
important element in determining our judgment as to the 
affinity of Limulus with Scorpio, we come to the considera- 
tion of a number of other structures, which we shall find 
some more and some less favorable to the hypothesis of a 
close relationship between the two animals. 
Connected with the exoskeleton and locomotor system is 
that remarkable development of an endoskeleton in the 
Arachnida, which Straus Durkheim put forward in the 
passage cited at the commencement of this article, as one 
of the leading characteristics of the class and one of the 
obvious features in which Limulus shows itself to be a true 
Arachnid. 
As already remarked, in speaking of the entapophyses and 
parabranchial stigmata, Limulus shows a marked tendency 
to the development of cartilage and fibro-cartilage by the 
modification of its connective substance at certain points and 
in certain areas. 
The most striking result of this tendency, is the formation 
of a large fibro-cartilaginous plate which lies in the cephalo- 
thoracic region between the alimentary canal above and the 
nerve collar below, and unconnected by hard parts with any 
portion of the exoskeleton. It is represented of the natural 
size as seen from the ventral (sternal) aspect in fig. 7, 
Pl. XXVIII. It has been previously figured by Van der 
Hoeven (12), whose figure is not very accurate, and is copied 
by Owen (7). be 
This body is the base of origin of a large number of 
muscles, and may be regarded as an enlargement and 
