VASCULAR AND REPRODUCTIVE SYSTEMS 449 
It is probable that the pigment spots between the lobes of the 
apertures, the tentacles in the branchial siphon, and the dorsal lamina, 
or its representatives, the languets, have some sensory function. ' 
Vascular system.—The simple tubular heart lies in a 
pericardial space at the ventral side of the lower end of the 
pharynx. In development, two diverticula grow out from 
the pharynx; these meet and fuse, forming the pericardium. 
The heart arises as an invagination from its dorsal wall, and 
is thus endodermal in origin, and probably not homologous 
with the heart of the other Vertebrates. A periodical 
reversal of the direction of the waves of contraction is 
discernible in the heart ; for a certain number of beats the 
blood is driven upwards, and then the direction is reversed. 
This same reversal also occurs in Phoronis. 
According to Herdman, the ventro-dorsal contractions occasion the 
following circulation :—The blood, which is spread out on the walls of 
the pharynx in vessels lying between the slits, collects into one large 
(branchio-cardiac) vessel, which, after receiving a vessel from the test, 
enters the ventral end of the heart. From the dorsal end it is poured 
into a great (cardio-visceral) trunk, which sends one branch to the 
test, and then breaks up among the viscera. From the visceral lacunze 
the blood is again collected (in a branchio-visceral) to be distributed 
to the branchial sac. At the reversal of the contractions this circulation 
is also reversed. The reversal occurs every couple of minutes or so. 
The blood is very colourless, but usually contains a few pigmented 
corpuscles. 
Excretory system.—In the loop of the intestine there 
lies a mass of clear yesicles containing uric acid and other 
waste products. This, therefore, seems to be a renal organ, 
lout there is no duct. Bacteria are usually found in the 
vesicles, and their activity may make diffusion easier. It is 
interesting to find such a plant-like method of storing up, 
instead of eliminating, waste products in these very passive 
animals. It has been suggested that the sub-neural gland 
may have some-renal function. 
Reproductive system.—Tunicates are hermaphrodite. 
The reproductive organs (Fig. 240, G.) are very simple, and 
lie in the loop of the intestine. The ovary is the larger, 
and contains a cavity into which the ova are set free, and 
from which they pass outwards along an oviduct which 
opens into the cloacal chamber. The testis surrounds the 
ovary, and is mature at a different time (dichogamy) ; its 
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