GEROULD: CAUDINA. 163 
determined that this statement is improbable, inasmuch as such a 
condition would necessitate the presence of three tentacles in every 
interradius, a condition at variance with that in other Molpadiidae ; 
furthermore it is obvious that Von Marenzeller made an examination 
of only the left half of the calcareous ring of Caudina arenata, as 
Ludwig (Zbid., p. 589) was on a priori grounds led to suppose. 
With the abundance of material at my disposal I have been able by 
direct observation to substantiate throughout the conclusions in re- 
gard to the conditions in C. arenata at which Ludwig has arrived, 
Histology.— As the external features of the tentacles were 
described in connection with the integument, I pass at once to their 
histology. "Phe tentacles are composed of five layers: (1) a colum- 
nar epithelium. covered with a eutieula, (2) a thin layer of connective 
tissue, (3) nervous tissue, forming a thiek band on the side of the 
tentacle next the axis of the body, and gradually diminishing in 
thickness on either side of the inner median line of the tentacle, (4) 
a layer of longitudinal muscle fibers, and (5) the internal epithelium. 
The external epithelium of the tentacles has already been described 
in connection with that of the body-wall. The connective tissue 
does not differ materially from that of the body-wall; calcareous 
bodies are, however, entirely absent. A thin hyaline membrane 
from this layer of the tentaeles overlies the layer of longitudinal 
muscle fibers here, as in the radial and circular canals, 
Circular muscle fibers, lying outside the layer of longitudinal ones, 
have been described by Danielssen and Koren (82) for the allied 
form Trochostoma; there are, however, no circular muscle fibers in 
the tentacles of Caudina, and most of the recent observers have been 
unable to confirm the earlier observations of Quatrefages (42) and 
Baur (64), that such fibers are found in Synapta. The internal 
epithelium is composed of flat, ciliated cells. 
Tentacular valves.— Valves, similar to the “Semilunarklappen ” 
which Hamann (83) has described for Synapta, are found in 
Caudina, Each valve is situated in a radial canal near its junction 
with a tentacular vessel (Plate 6, fig. 77) ; it is attached to the calca- 
reous radial plate which forms the outer wall of the canal and, to 
some extent, to the connective tissue of which the side wall is 
composed, The valve consists, as in Synapta, of muscle fibers, 
radial to the chief axis of the body, which are surrounded, especially 
on the attached edge, with fibers of eonneetive tissue; by the 
contraetion of these muscle fibers the valve is drawn aside from the 
