302 MR. NEWPORT ON THE MYRIAPODA AND MACROUROUS ARACHNIDA. 
and limbs ; and backwards, through the spinal artery into the tail ; the returning 
blood being collected by the portal vessels and sent to the branchiae, from whence it 
is again transmitted to the heart. This is the general course of the circulation in the 
macrourous Arachnida, so that in these there is a current backwards along the great 
dorsal artery, and the heart propels its fluid in every direction by the successive 
actions of its chambers. 
3. The development of the nervous and circulatory systems is shown to be effected 
by two modes ; first that of growth, extension , or enlargement of individual parts ; of 
which mode the elongation of the cord in its gangliated portions is evidence ; next, 
that of aggregation of two, or more parts, of the same general structure, to form par- 
ticular regions, or divisions of that structure. Of this latter mode, to which the term 
development has usually been restricted, the union of two or more originally separate 
segments of the body, as in the changes of insects, of two chambers of the heart, to 
form a single chamber, as in the iulidse, and the coalescence into one mass of two 
ganglia and their cords, in which the first mode of development, by simple growth, 
has been completed, are examples, as in the pseudo-changes of the Myriapoda, and the 
complete metamorphoses of insects. This latter mode of development usually takes 
place when the former has been carried to its fullest extent, and is induced by changes 
in the external structures of the body, being entirely peripheral in its origin. It is not 
restricted to those animals which undergo a complete metamorphosis, but also takes 
place, to a greater or less extent, in those in which changes are scarcely perceptible, 
and in which the first mode of development chiefly prevails. It is also in operation 
in the first-formed parts of the body, while that of extension is predominant in the 
latter ; as in those beings in which the body is formed of a multitude of similar struc- 
tures successively produced ; as in the Myriapoda, in which it is least predominant in 
the lowest forms, the Iulidse, but is carried to its greatest extent in this class in the 
highest, the Scutigeridce ; while in another class, the Arachnida, it has attained its 
maximum in the earliest conditions of the animal, even before it has escaped from 
the ovum. 
