108 E. RAY LANKESTER, ON THE 
body four principal modifications of the circulation, the 
simplest of which is that extending from the 19th segment 
to the penultimate one, and represented diagrammatically in 
figs. 1 and 8. The seven cephalic segments, and the one 
caudal, in which the circulation is merely capillary, all the 
great trunks being broken up, form a second modification, 
whilst the third is that described above in the generative 
segments, and represented diagrammatically in fig. 2; whilst 
a fourth modification occurs in the 15th, 16th, 17th, 18th and 
19th segments, where the deep commissural vessels are large 
and single, but there are no additional parallel vessels. 
Structure of the Vessels —The vessels, thus disposed for the 
purpose of circulating the red fluid, may be considered as 
possessing an internal structureless amorphous tunic, without 
epithelium, and an external tunic of more or less modified 
connective tissue; between these two are longitudinal and 
transverse muscular fibres in some of the smaller as well as 
in the large vessels.* In most of the vessels the transverse 
fibres are radiated from a point and placed in bundles at 
intervals (fig. 7). The result is that when the transverse 
fibres contract they produce an uneven moniliform appear- 
ance in the blood.vessels, but are the more effective in pro- 
pelling the fluid. The alternating pomts of contraction anil 
expansion in the dorsal vessel and’so-called hearts are well 
seen when a worm is freshly opened. 
Structure of the Vascular Fluid.—The vascular fluid is 
completely devoid of corpuscles, and is entirely structureless. 
It is more easily coagulated than the perivisceral fluid, but 
otherwise appears to have the same composition. The nature 
of its colouring matter is not known. 
Functions and homologies of the Vascular and Perivisceral 
Fluids.—It has been already pointed out that De Quatre- 
fages has established the existence of undeniable homological 
relations between the perivisceral fluid of the Annelida and 
the fluid occupying sinuses and lacune among the Crustacea, 
Arachnida, and Insects,} and circulated by a heart with valves, 
and considered as true blood. The researches of Professor 
Huxleyt tend to establish the conclusion that in the vascular 
system we have a closed representative of the water-vascular 
system of Scolecida. The function of the two fluids does 
not in any way necessarily affect the question of their 
homologies, and in considering the part which they play 
in the animal economy we must not be hampered by the 
* See Leydig’s ‘ Lehrbuch der Histologie.’ 
+ ‘ Annales des Sciences Nat.,’ 1843—1854. 
t ‘ Brit. Ass. Reports,’ 1854. 
