350 A. T. MASTERMAN. 



plumes — and, in the case of the two lateral skeletal masses, 

 forms a fulcrum for the pre-oral muscles. These lateral 

 masses, by their structure, origin, and function, can be 

 homologised with the chondroid tissue of Balanoglossus 

 (Marion). 



Vascular System. — There is no prior reference to a vas- 

 cular system. A system of sinuses, bearing a close similarity 

 to that of Actinotrocha, is capable of easy demonstration. 

 The walls of the sinuses are in all cases formed by the meso- 

 derm of the coelomic cavities, and in some cases either the 

 ectodermal or endodermal wall. In certain parts, especially in 

 the dorsal vessel, there are thickenings of the walls caused by 

 nucleated mesodermic cells which bear a close likeness to those 

 described in Actinotrocha, and it can hardly be doubted 

 that these are contractile as in the latter. Immediately under 

 the nerve-ganglion lies a large sinus, which, as in Actino- 

 trocha, we may term the subneural sinus. It is very con- 

 stant in outline (figs. 2 and 3), and has, in transverse section 

 of the epistome, a quadrilateral shape. Its dorsal wall is 

 formed by the nerve-ganglion, and the other three sides of the 

 rectangle (i.e. laterally and ventrally) are formed by the 

 coelomic wall of the epistomial cavity. Anteriorly it is blind, 

 and posteriorly it tapers off into the mid-dorsal blood-vessel, 

 the greater part of which is filled up by the long csecal sub- 

 neural gland [s.n.g.) (see below). Its course can be followed 

 in the series of sections from fig. 1 to fig. 9, in w^hich it is 

 tinted red. The dorsal blood-vessel in this region [d. b, v.) is 

 bounded laterally by the collar walls, and these are thickened 

 and, as already mentioned, probably contractile. 



At the level of fig. 10 this vessel separates into two, which 

 encircle a part of the gut to be referred to later, and again 

 join up beyond this to form a single median vessel (fig. 15, 

 d.h.v.). Dorsally to the pharynx this vessel can be traced 

 (fig. 16) till it reaches the stomach, round which it breaks up 

 into a system of sinuses, which, more or less disconnected in 

 the preserved specimens, is probably one large blood-sinus in 

 the living animal. From this sinus branches, with indications 



