1382 THE VOYAGE OF H.M.8, CHALLENGER. 
Pentacrinid are very variable in this respect, some individuals remaining fixed through- 
out life; while others become detached and are henceforward more or less free like 
the Comatulze, attaching themselves temporarily by means of their cirri. 
Some Comatulz, however, lose their cirri altogether when mature. The older ones 
gradually drop off without beimg replaced, while their sockets become obliterated until 
nothing remains of the centro-dorsal but a flat plate in the centre of the radial pentagon. 
This is the case with the Actinometra jukesi, Actinometra stellata, and the fossil Actino- 
metra lovén. Some specimens of Millericrinus pratti reach the same condition, nothing 
remaining of the stem except a pentagonal disk in the centre of the ring of basals ; so that 
in the absence of other stalked individuals these would naturally be taken for Comatule. 
This suggests the question whether the single plate in the centre of the calyx of Mar- 
supites and Uintacrinus may not be really a separated top stem-joint, and not a dorso- 
central plate homologous with that of Urchins and Stellerids as is generally supposed. 
It is noteworthy that certain Blastoids, e.g., Eleutherocrinus and Astrocrinus, were 
stemless and free just like Marsupites; and it is possible that the same was the case with 
some species of the Palzeozoic genus Agassizocrinus, at any rate in later life. Other 
Palocrinoids, together with some Blastoids and Cystids, must have been almost equally 
free, as has been already explained in Chapter II. For though a stem was present, it 
was often quite short, and almost or entirely devoid of cirri; though it was sometimes 
fixed by coiling itself round other Crinoid stems and similar bodies. 
The apparently perfect freedom of many of these forms is very singular and difficult to 
understand. Much would be learnt about them, no doubt, if the habits of a living Acti- 
nometra jukest were carefully watched, for it is well established that Comatule which 
have once anchored themselves by their cirri remain so fixed for a considerable time, 
except perhaps at the period of sexual activity; and it would therefore be very interesting 
to know how far the cirrus-less forms remain permanently fixed. Seaweeds, Polyzoan 
colonies, Corals, and Zoophytes often serve as the anchorage of Comatule. Thus in one 
case that I have met with the cirri were coiled round a Dendrophyllia, and in another 
round the stem and branches of a Gorgonia; while it sometimes happens that the cirri 
of a Comatula are fastened round the still larger cirri of a Metacrinus or Pentacrinus. 
The food of a Crinoid is considerably varied in its nature according to the character 
of the sea-bottom on which it lives. The horny casings of Entomostraca and the larvee 
of larger Crustacea are frequently to be found in the digestive tube, together with the 
frustules of Diatoms, spores of Algz, &c. Dr. Carpenter mentions Peridiniwm (Ceratium) 
tripos, Ehr., as a principal article of food of the Arran Comatulz ; while in sections of 
Bathycrinus, Rhizocrinus, and Pentacrinus from deeper water the siliceous shells of 
Radiolarians may be found in considerable abundance and variety. Foraminifera too 
form a staple article of food for these deep-sea species. I have frequently found Globi- 
gerina, Biloculina, and other types beneath the covering plates of the food-grooves on 
