Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/659

637 E C H I N D E R M A T A 637 subtentacular canals; gr, genital rachis ; cc, coeliac canal ; in, muscles ; a, organic basis of calca reous segment; 6, solid cord from quinquelocular organ. (fig. 25), the tentacles communicate at their bases with a common trunk, the tentacular canal. Beneath this, but having no communication with it, lies the subtentacular canal, which is usually divided by a more or less incomplete septum. Each of the subtentacular canals is con tinuous with a branch of the axial canal, which communicates with the deeper portion of the perivisceral cavity by means of a minute pore situated nearly at the centre of the lower surface of the visceral mass, and partly occupied by the pedicel before referred to. As the axial canal ex tends downward through the visceral mass it comes into contiguity with the FlG- 25. Section of alimentary canal, and opens into it by arm of Antedon rosa- irregular passages. There is in the ceus - (After Carpen- arms a third canal, the coeliac, which ter y . ., . , . tc, tentacular canal, giving IS a Continuation OI the body-Cavity off lateral branches to or ccelom, and is separated from the ^ a ^^^,^ e the &amp;gt; ^wo subtentacular canal by a transverse partition. At the junction of this partition with the septum of the sub- tentacular canal there is a passage, the gtnital canal, in which lies the cellular cord known as the generative rachis, in connection with the visceral genital tissue. Enlargements of the rachis in the pinnules constitute the genital glands of the Crinoidea, the products of which may or may not be dis charged by special orifices. Towards the extremity of the pinnules, in Antedon, the partition between the coeliac and subtentacular canals thins away, and becomes finally obso lete, thus possibly affording a means for the circulation of the nutritive fluid of the body, the subtentacular canals constituting an arterial or distributive and the coeliac a venous or collective system of vessels. From the arms the tentacular canals proceed inwards to unite with a circular canal situated around the gullet, and having connected with it numerous short processes similar t:&amp;gt; the vasa, ambulacralia cavi of the Ophiuridea (see page 635). There is no madreporic tubercle, and the madreporic canal is apparently unrepresented in the Cri noidea. Respiration seems to be effected by the tentacles, and in Comatula also by the access of water through pores in the oral perisoine, communicating with a series of sinuses below its under surface by means of funnel-shaped canals. The central organ of the nervous system in Antedon, accord ing to Carpenter, is the dilated cortical portion of the axis of the stem within the centro-dorsal plate, which supplies branches to the cirri and the arms, and corresponds pro bably with the axial sheath which, in Pentacrinus, sends off cords at the nodes of the stem into the whorls of cirri (see fig. 21). A fibrillar band underlying the epithelial floor of the brachial furrows is regarded by Ludwig as a nerve &quot; an afferent rather than a motor nerve&quot; (Carpenter). The development commences with the formation from the egg of an oval morula, which acquires four hoop-like ciliated bands, and a posterior terminal tuft of cilia. An cndodermal sac or archenteron results from an invagination of the blastoderm between the third and fourth ciliated bands; and from this three diverticula, two lateral and one ventral, take their rise, the remainder of the archenteron becoming an alimentary cavity communicating with an anterior oesophagus. The lateral diverticula are transformed into peritoneal sacs, one on the dorsal the other on the ventral side of the alimen tary cavity, and their walls coming in contact produce a circular mesentery. In the ventral diverticulum the ambulacral vessels have their origin. Around the alimentary cavity, when the pseud-embryo is scarcely a line in length, there are formed two circles, each of five calcareous plates, which eventually become the oral and basal ossicles of the calyx. From the centre of the posterior circle extends a row of eight calcareous rings, the future stem of the Crinoid, surrounding a backward prolongation of the dorsal peritoneal sac. At the posterior extremity of the row is a cribriform disk, by which the young Crinoid subsequently attaches itself. The sarcodic body of the pseud-embryo begins to shrink, the pseudostome and the two lower bands of cilia disappear, and afterwards the two upper bands, and the embryo then becomes fixed to a stone, seaweed, or some other object. A new mouth is formed in the centre of the disk by the separation of the oral plates, and the intestine by the production of a diver ticulum of the alimentary cavity. In the early Pentacri noid stage of Comatula, the basals rest upon the centro- dorsal segment, but become at length metamorphosed into a single piece, the rosette; and the centro-dorsal segment by degrees increasing in size, the first radials come to rest upon its enfolded lip. During the same period, after the forma tion of an anus, the oral and basal plates disappear. The development of the dorsal cirri takes place as the proximal joint of the column enlarges to form the centro-dorsal _ piece. At the end of five or FlG - 26. -Pentacrinoid larval forms of six months, when about an Comatula, natural size and magm- inch in diameter, the young Comatula detaches itself from its stalk, and is then able to swim by means of its arms. The Pentacrinoid larval form of Comatula (fig. 26), previous to his discovery of the ultimate stages of its growth, had been termed by Vaughan Thompson Pentacrinus europccus. The Crinoidea are classified as follows : Order I. TESSELATA. Calyx completely formed of calcareous plates, oral face without ambulacral furrows. Family. Tesselata. Ex. C yathocrinus, Actinocrinus. Order II. ARTICULATA. Oral face of calyx usually membranous or sub-membranous, with ambulacral furrows. Family 1. Pentacrinidce. Always attached. Ex. Pentacrinus, PJdzocrinus. Family 2. Coniatulidce. Attached only in the young state. Ex. Antedon, Phanoc/enia. The Cretaceous genus Marsupites appears to have been unattached. The Crinoidea are represented by Glyptocrinus, Eucalyptocrinus, Marsupiocrinus, Taxocrinus, Iclithyocrinus, Periechocrinus, Cupres- socrv ius, Poteriocrinus, Woodocrinus, Cyathocrinus, PJiodocrinus, FIG. 27. Fossil Crinoidea. 1. Crotalocrinus rugosus, Mill. ; U. Silurian, Dudley. 2. Poteriocrinus (joint of column) ; Carboniferous, Yorkshire. 3. Encrinus entrocha; L. Musclielkalk, Germany. 4. A piocrinus Parkinson!, Mill.; Bradford Clay. 5. Pentacrinus basal tiform is, Mill.: Has, Lyme. 6. Mnrsupites ornatus, Mill. ; Chalk, Sussex. 7. Comatula Glenotremites (upper surface of body). 8. Comatula (lower surface) ; Chalk, Sussex. 9. Eugeni:icrinus quinquedaetylus, Schl.; Oxfordian, Wiirtemberg. 10. Bonrgueticrinus ellipticus, Mill. ; C/iali; Kent. and numerous other genera in Paleozoic strata, where their remains, especially in the Carboniferous series, are often the chief constituents of vast masses of compact limestone. 1 From their form the insulated articuli of the stem have come to be known as entroclii, scrciv- stones, or wlicel-stones, and in the north of England, as &quot; St Cuth- 1 On Criuoidal Limestone, see J. Eofe, F.G.S., Geol. Mag., x. p. 262.