SKELETON VASCULAR SYSTEM 



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 " 1 

 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, r), 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 lies 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 (n) 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 AVilley. 



