I SKELETON—VASCULAR SYSTEM 15 
of Balanoglossus, and in most species it grows merely by the 
deposition of laminae of chitin from the notochord, and from the 
ventral epidermis of the proboscis-stalk. 
In some species, however, and particularly in Balanoglossus 
aurantiacus and Glandiceps, the primary skeleton becomes sur- 
rounded by an extensive development of a secondary cartilaginoid 
skeleton, consisting of a structureless substance into which the 
adjacent body-cavities of the proboscis and collar send cellular 
outgrowths. The possibility of a relation between this tissue, 
more or less surrounding a part of the notochord, and the carti- 
lage of Vertebrates cannot be overlooked. 
The caudal region may be stiffened (?) by a “ pygochord”? 
which is a median derivative of the alimentary canal on its 
ventral side. 
Vascular System and Proboscis-Gland.—The main vessels 
are a dorsal and a ventral vessel (Fig. 4, d.v, v), lying in their 
respective mesenteries. The details of the vascular system are 
complicated, and have not been thoroughly made out, the nearly 
colourless character of the blood making their investigation a 
difficult matter. The following points may, however, be noted. 
The blood is said to pass forwards in the dorsal vessel, which, 
like the ventral vessel and a pair of lateral vessels in the hepatic 
region, is contractile. In the collar the dorsal vessel hes be- 
tween the two perihaemal spaces, on the dorsal side of the base 
of the notochord. The principal blood-space in the proboscis 
(Fig. 7, >) lies between the notochord (nm) and an organ known 
as the “heart-vesicle” or “pericardium” (p.c). The latter has 
muscular walls and it contracts rhythmically in the larva. Its 
behaviour in the adult is not so easily made out, but it is prob- 
able that, although it does not communicate with the vascular 
system, its contractions propel the blood contained in the space 
immediately beneath it. The blood, after passing to a glandular 
organ, the “ proboscis-gland” or “ glomerulus,” which lies at the 
sides and in front of the notochord, appears to pass round the 
collar to the ventral vessel. Various systems of vessels are con- 
nected with the skin, the gills, the alimentary canal and the 
generative organs. 
The function of the proboscis-gland is possibly excretory. 
In this case it is probable that the proboscis-pore eliminates the 
1 Willey. 
