132 THE VOYAGE OF H.M.S. CHALLENGER. 



Pentacrinidse are very variable in tliis respect, some individuals remaining fixed through- 

 out life ; while others become detached and are henceforward more or less free like 

 the Comatulse, attaching themselves temporarily by means of their cirri. 



Some Comatulse, however, lose their cirri altogether when mature. The older ones 

 gradually drop off without being 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 jtikesi, Actinometra stellata, and the fossil Actino- 

 metra loveni. 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 Comatulse. 

 This suggests the question whether the single plate in the centre of the calyx of Mar- 

 siipites and Uintacrinus may not be really a separated top stem-joint, and not a dorso- 

 central plate homologous ■wdth 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 Palaeozoic genus Agassizocrinus, at any rate in later life. Other 

 Pala30crinoids, 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 jukesi were carefully watched, for it is well established that Comatulse 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 Comatulse. Thus in one 

 case that I have met with the cirri were coiled round a DendrophyUia, 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 Pentacrimis. 



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 larvse 

 of larger Crustacea are frequently to be found in the digestive tube, together with the 

 frustules of Diatoms, spores of AlgSB, &c. Dr. Carpenter mentions Peridinium (Ceratium) 

 tripos, Ehr., as a principal article of food of the Arran Comatulse ; while in sections of 

 Bathycrinus, Rhizocrinvs, and Pentacrinus from deeper water the sihceous shells of 

 Kadiolarians 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 Glohi- 

 gerina, Biloculina, and other types beneath the covering plates of the food-grooves on 



