434 Mr. Newport on the Anatomy and 
furca (w), the rudiments of an internal or endo-skeleton, to which the princi- 
pal muscles of the segments and. organs of locomotion, the legs, are attached, 
and which partially enclose and protect the nervous cord and ganglia, like 
the rings of vertebrze in the Vertebrata. Some traces of the entrances into 
these fureal bones exist in the sternal plates of Perla, but they are flattened 
and quite unlike those of Pteronarcys. 
Spiracles.—But although the sternal orifices do not communicate with the 
respiratory organs, the imago Pferonarcys most certainly is endowed with an 
aérial as well as a branchial form of respiration. It has three pairs of large 
thoracic spiracles of most complete structure, which are situated in the places 
usually oceupied by these organs in other insects, and which are covered in 
the pupa of this insect by branchiz. The first pair (figs. 6, 7, 8) is in the 
tegument which connects the pro- and mesothorax, the second in the junction 
of the meso- and metathorax behind the first pair of wings, and the third is 
in the anterior of the segment immediately behind the metathorax, at the 
base of the second pair of wings. The segment which bears the latter pair 
bears also on its under surface a pair of branchize, like the true thoracic seg- 
ments, and ought perhaps to be regarded only as part of the metathorax in- 
stead of a distinct segment, the first abdominal. Besides the thoracic spi- 
racles there are also a series of false abdominal ones, one pair at the sides of 
each segment. These are situated at the precise spot occupied in the second 
abdominal segment by the last pair of branchiæ (fig. 5b). They are enclosed 
by a circular elevation in the tegument (fig. 9) and have an imperfect vertical 
valvular opening, which leads into a small cavity that is closed internally by - 
à cribriform membrane by which the spiracle is separated from the cavity of 
a large trachea that is connected with it. These spiracles therefore are me- 
diate in structure between the branchial and aérial form of organ, and re- 
semble those which I formerly described in the Transactions of this Society * 
as common to a genus of Myriapoda, the Heterostoma, ‘The thoracic spi- 
racles al teronarcys, on the contrary, are most complete structures. The 
three pairs are all similar in formation, the second, or mesothoracic, being 
somewhat. the largest, They are placed vertically in the flexible tegument 
between the segments, and open and shut by a double valve.. The pro- 
* Vol. xix., Monograph of the Class Myriapoda, p. 413. 
