118 PKOF. A. DENDY AND ME. E. HINDLE : ADDITIONS TO 



The madreporic canal is nearly straight and ends in an inconspicuous 

 madreporite. Its length is about 1*3 mm. and transverse diameter O'lS mm. 

 Its wall consists o£ three layers : — (i.) An inner layer o£ columnar cells 

 which are much longer on the side remote from the dorsal mesentery 

 than on the side to which it is attached (fig. 24) ; (ii.) a connective- 

 tissue layer, from which calcareous deposits are absent ; (iii.) a layer of 

 peritoneal cells. 



The Polian vesicle is a comparatively large thin-walled sac of an elongated 

 ellipsoid shape, opening into the circular canal on its ventral side. It is 

 usually about 2-5 mm. long and 0-5 mm. in thickness at its widest part. The 

 wall appears to contain the same layers as the circular canal, but they are 

 thicker owing to the greater development of the muscular and connective- 

 tissue layers. 



The ten tentacular canals arise from the anterior side of the circular canal. 

 They run forward on the pharynx internal to the calcareous ring. Opposite 

 the anterior part of the ring they widen considerably, and each canal gives 

 off a blind diverticulum which runs backwards over the outer side of the 

 calcareous ring and is very closely appHed to it (fig. 17, d, and fig. 0, d). 

 The calcareous ring therefore appears to be partially enclosed in the tentacular 

 canals, as shown in fig. C and in PL 14. fig. 23, the walls of the tentacular 

 canals themselves and of their backward diverticula being so thin where they 

 touch the ring as to be inconspicuous. From the position of these diverticula 

 it seems possible that they may represent the remnants of tentacular ampullae, 

 for when present these are also frequently attached to the calcareous ring, for 

 a short distance at least. Near its base, and opposite to the diverticulum on 

 the other side of the calcareous ring, each tentacular canal shows in section 

 a couple of longitudinal septa (fig. C, r.) which doubtless represent the valves 

 described by Hamann in Synapta *. 



The tentacles exhibit five layers in their walls (fig. 4) : — (i.) An external 

 columnar epithelium, the cells of which are very long, tapering internally, 

 and are covered at their outer ends by a thin layer of cuticle. These cells do 

 not differ from those described under the heading of integument, except in 

 being longer (50-60 fi) on the outer side of the tentacle, (ii.) A connective- 

 tissue layer which varies considerably in thickness, but is usually thickest on 

 the outer side of the tentacle. It has the structure of the connective tissue 

 of the integument, containing connective-tissue cells, stellate cells, and 

 scattered fibres, with an internal nervous layer ; in addition it contains a 

 great number of peculiar, large and frequently multi-nucleate cells of 

 doubtful nature, (iii.) A very thin layer of circular muscle-fibres, (iv.) A 

 layer of longitudinal muscles supported internally by a basement membrane. 

 This layer is thickest in the stem of the tentacle, gradually diminishing in 



* Hamann, Beitrage zur Histologie der Echmodermen. Die Holothurien, p. 33. 



