INTERNAL ANATOMY 251 



appear to be torn away from their attachments in the dorsal wall of the collar (PI . XXXVI I , 

 fig- i>P^)- It is therefore probable that these are dilator muscle fibres which widen the 

 lumen of the collar pores when water is drawn in, the connection of some of the fibres 

 having been torn away by vigorous contraction when the zooids died. 



VASCULAR SYSTEM 



The vascular system of Cephalodiscus was described first by Masterman (1898 b, 

 p. 350). He observed that it is composed of a pericardium situated on the tip of the 

 notochord, a dorsal vessel running downwards in the mesentery of the collar, a system 

 of sinuses round the stomach, vessels to the ovaries, a ventral vessel which divides into 

 two at the base of the notochord and passes round the mouth to the anterior side of the 

 body and stolon, vessels on the dorsal side of the body cavity of the arms and dorsal 

 and ventral vessels in the stolon. He stated that the vascular system of Cephalodiscus is 

 identical with that of Balanoglossus . The dorsal vessel communicates with the heart, and 

 glomeruli occur in the ventral wall of the pericardium and in the ventral blood sinus. 

 He also described a blood sinus running in the dorsal wall of the proboscis which was 

 connected with the anterior glomerulus. Harmer (1905, p. 75) denied the existence of 

 certain of the vessels described by Masterman. It was difficult to demonstrate that the 

 dorsal and ventral vessels of the stolon were connected with similar vessels in the trunk ; 

 so he distinguished the spaces in the body as [a) vessels, the vascular nature of which 

 was very clear, and {b) doubtful spaces. He recognized (i) the existence of the dorsal 

 vessel which starts from the anterior caecal projection of the stomach and runs upwards 

 parallel to the pharynx. This vessel, he beheved, functioned as a reservoir of blood. 

 He recognized also (2) vessels in the lateral mesentery passing ventral to the oviduct into 

 the ovary and supplying it with nutriment, (3) two vessels in the stolon, the anterior 

 vessel passing into the anterior part of the ventral mesentery and the posterior vessel 

 passing into the ventral mesentery as far as the alimentary canal. To this detailed account 

 there are only a few additions to make, and they are based only on the sections of C 

 densus. The space inside the heart as well as all the discernible vascular spaces are deeply 

 stained and so it is easy to distinguish them from non-vascular spaces in the body. 



In C. densus the heart is not situated on the tip of the notochord as in C. dodecalophus 

 but on its ventral surface, so that the pericardium extends backwards almost parallel to 

 the notochord. The heart is thick-walled and formed by the invagination of the posterior 

 wall of the pericardium. The space inside the heart is dilated and deeply stained 

 (PI. XXXVII, figs. 1-4). This communicates directly with two blood vessels. A blood 

 vessel originating from the dorsal side of the heart passes round the tip of the notochord, 

 gives off a branch which runs forwards to the dorsal wall of the proboscis and proceeds 

 backwards between the collar cavities close to the dorsal side of the notochord. Glomeruli 

 occur on its walls in the hinder part, where it is very thick. On reaching the roof of the 

 dorsal diverticulum of the pharynx it divides into two. These branches which I propose 

 to call the pharyngeal blood vessels (PI . XXXVI I , fig. 2 , pv^^ and pv<^ pass backwards close 



