1895 - 96 .] Mr Masterman on the Structure of Actinotrocha. 131 
teries running from the base of the most posterior tentacles upwards 
and inwards to meet the oesophagus, and by paired mesenteries 
from the pre-oral cavity running in front of the first anterior 
tentacle inwards and downwards to the oesophagus (fig. 2). 
Under the apical plate, the collar walls and the pre-oral meso- 
dermic wall do not come in contact, but leave a haemocoele space. 
The dorsal and ventral blood-vessels and the paired lateral 
vessels are also haemocoelic spaces between the gut-wall and the 
mesoderm of the collar and trunk. 
trocha, to one side of 
median line. 
Lastly, a similar haemocoelic space forms a ring sinus just below 
the peri-anal ciliated band (fig. 1). There is one pair of nepliridia 
which consist of ciliated tubes opening to the exterior on the 
ventral side below the ring of tentacles. The tubes run between, 
the collar and trunk walls, and protrude into the collar cavities. 
They terminate in a branched internal funnel closely resembling 
that described for Amphioxus by Prof. Boveri. The apertures 
appear to open into the collar cavities, a fact which, if confirmed, 
would enable them to be homologised with the collar pores of 
Balanoglossus . 
