GEROULD: CAUDINA. 165 
muscle fibers and, finally, the inner epithelium, which differs from 
that of the rest of the water-vascular system in that the cells are 
thinner and flatter than elsewhere. 
The paired rudimentary ambulacra present histological conditions 
similar to those of the ‘ambulacral canals of holothurians generally. 
Their walls (Plate 6, fig. 79) consist of a layer of longitudinal 
muscles enveloped by a structureless hyaline membrane, and, inside 
the muscles the ordinary epithelium lining the water-vascular system. 
The fact that the anal papillae of Molpadiidae are, in some cases 
at least, ambulacra was suspected by Ludwig, who, in stating that 
ambulacral organs are lacking in the Molpadiidae, adds in a foot- 
note (89-92, p. 100): “Ich kann den Verdacht nicht unter-driicken 
‘dass die fiinf kurzen, etwas iistigen Papillen’, welche Semper an der 
Kloakenéffnung seiner Haplodactyla molpadioides beschreibt und 
abbildet, sich bei eingehender Untersuchung als umgewandelte 
Fiisschen herausstellen werden.” 
Anal papillae, as is well known, are of quite general occurrence in 
the Molpadiidae. All species of the genus Haplodactyla have been 
shown to possess anal papillae, except the imperfectly known H. holo- 
thuroides Cuv., which Théel regards as identical with H. australis. 
In the genus Trochostoma the presence of anal papillae in T. 
arcticum, T. boreali, and T. Thomsonii, has been shown by Da- 
nielssen and Koren (’82), in T, albicans and T. antarcticum by 
Théel (82), and in T. granulatum and T. intermedium by Ludwig 
(94) ; thus they have been found in seven out of fourteen well- 
authenticated species of this genus. The presence of five anal 
papillae in the genus Ankyroderma has been given by Danielssen 
and Koren as a characteristic of that genus. They have, at all events, 
been shown to exist in every species classed in this genus, except A. 
limicola (Verrill), A. Marenzelleri (Théel), A. Roretzii (v. Marenz.), 
and A. spinosum (Ludwig). In the genus Caudina, anal papillae 
were not found in C. caudata (Sluit.), nor in C. Ransonetti (v. 
Marenz.). Théel describes in C. coriacea (Hutton) five groups of 
anal papillae with 5—7 papillae in each group. In Ludwig’s C. cali- 
fornica the tail unfortunately was injured. Thus, if anal papillae are 
present throughout the genus Caudina, they are so small in both C. 
caudata and C. Ransonetti as to have eluded observation, just as 
they have hitherto done in C. arenata. 
Among the Holothuriidae anal papillae are described in Bohad- 
schia ; among the Cucumariidae five groups of feet around the anus 
