1895 - 96 .] Mr Arthur T. Masterman on Plioronis. 
67 
The main features are : — Simple epidermis, with mucous glands and 
nerve fibrils, concentrated dorsally in the median line ; a dorsal 
and ventral mesentery, the former incomplete in the hind region, 
as in some species of Balanoglossus ; dorsal afferent blood-vessel in 
dorsal mesentery, communicating with the ventral efferent vessel 
by enteric sinus ; ventral efferent vessel connected with ventral 
mesentery, and dividing into two in collar ; coelomic pouches, 
largely filled with connective-tissue, sexual glands, a thin layer 
3 . 
of circular muscles, and within them a layer of longitudinal 
muscles, inserted anteriorly in chondroid tissue, posteriorly in 
body wall ; cuticular secretion of ectoderm, forming more or less 
permanent “ tube.” 
I would define the Hemicliordata as follows : — 
Ccelomate animals, with three parts of the body, an impaired pre- 
oral and two paired post-oral, each opens typically to the exterior 
by ciliated pores ; ectoderm simple, with mucous glands, partially or 
wholly ciliated ; nervous system ectodermal, concentrated in dorsal 
collar region, at base of epistome, and in post-oral collar ring — there 
may or may not be dorsal or ventral cords or both ; simple black or 
coloured pigment in the ectoderm cells — a cuticular secretion of the 
