386 
E. RAY LANKESTER. 
in mind, it becomes probable that the branchial apparatus as 
we see it is modified as compared with an earlier condition in 
which the blood-vessels played a more prominent part, and 
were more largely and distinctly developed throughout the 
organism. The probably degenerate condition of the vascular 
system in Amphioxus has led me to doubt whether the spaces 
marked Bl. vess. in figs. 1 and 2, PI. XXXVI B, are really 
continued as distinct vessels to the dorsal aortse; it is not 
unlikely that such a continuation exists, but it is also not 
unlikely that the original branchial vessels have effected a 
communication with the coelom. The generalisation that a 
fragmentary vascular system is not in a primitive condition 
but is in a state of degeneration appears to be warranted by a 
survey of vascular systems in the animal series, and by the 
a priori argument that a vascular system must be efficient as 
a circulating and distributing apparatus in order to afford the 
advantage necessary for the operation of natural selection. 
The probable steps of the primary or ascending evolution of 
a vascular system do not include a condition in which large 
vessels are present without capillaries or are in free com- 
munication with the coelom. 
From the examination of the pharyngeal bars we may now 
proceed to that of the median ventral tract of the pharynx, 
which it is convenient to call in toto the “ endostyle,” the 
name being justified by the undeniable identity of the peculiar 
median ridge of epithelium with that which is recognised by 
this name in the Ascidians. 
Below the endostylar epithelium, as shown in the series of 
sections, figs. 4 — 9, PI. XXXVIiJ, there is a chitinous plate 
which has hitherto remained undescribed. It consists of 
right and left moieties, and is segmented; that is to say, it 
thins out and disappears for a brief space at intervals. This 
endostylar skeleton, in fact, consists of a number of pieces 
following one another, corresponding in number to the primary 
gill-slits, each piece being composed of a loosely-joined over- 
lapping right and left half. The endostylar skeletal plates 
yest on the ends of the chitinous arches formed by the union 
