History of British animals - University of Guam Marine Laboratory

494 RADIATA. CRINOIDiE. Pentacrinus.foreign bodies from a ;depression in the centre of this plate arises the filiformcolumn, slightly incrassated towards the summit, and capable of bending ortwisting, composed of about twenty-four joints tbe external form and internalstructure of the column not ascertained. Axillary side arms in a single;row around the last joint of the column, corresponding with each of the costals,revolute, composed of about ten joints, the terminal one hooked. Plates of thepelvis too obscure to be determined. The first costals are thick, wedge-shaped,broadest upwards, the distal extremity emarginate, and supporting a secondcostal of a narrow subconical form, truncated and emarginate where it joinsthe scapula. The summit of the scapula is angular, having a sloping face oneach side, for the attachment of the first arm joints. The arms are ten innumber, and being supported on scapulae greatly narrower than the first costals,are rather remote at their base. The arms consist of about twenty -foursolid joints, tapering towards the extremity, simple in youth, but with ageand size dividing and subdividing ; along the side a row of dark spots each;arm supports on each side, in an alternate order, a row of annulated cylindrical,flexible, fleshy processes, covered with capitate filaments. The armsin these respects resemble those of the Comatula, in which I have observed asimilar row of brown tubercles with a granular surface, a fleshy productionon the ventral margin of the arm, broadest at the middle, supporting cylindricalprocesses, fringed with smaller filaments, ending in an expandedsucker-like Extremity. Within the costals, the body is subglobular,in thecentre of which is the mouth, formed by five petal-like valves, within whichare several soft tentacula ;without the valves is an opening with a tubularextensile margin, supposed to be the vent. In the early stages of its growthneither column nor arms are visible, the body appearing like a little club, fixedby an expanded basis, and giving exit, at its apex, to a few pellucid tentacula.The existence of the second aperture in the body of the genus Comatula, whichMr Thompson, in the interesting paper from which we have extracted the descriptionof this species, supposes not to have been previously observed, wasknown to Mr Adams thirty years ago, and described by him in the LinneanTransactions published in "1800. In the Annals of Philosophy for Nov. 1806,Mr Gray likewise announced the existence of this second aperture, withoutbeing aware of its previous detection. If£future observations, on better preservedspecimens, shall confirm the results of Mr Gray's examination of therecent Pentacrinus caput Medusa:, and demonstrate that it has but one openingto its alimentary canal, it will be necessary to constitute this species, discoveredby Mr Thompson, into a new genus, which may be termed Hibernula(the forerunner we hope of many important additions to our Fauna,when the shores of Ireland shall have been more extensively explored), andby characterising the family Comatulad^e, as possessing,besides numerousother common properties, an alimentary canal with two openings, it will benaturally divided into two sections, the first containing the fixed Hibernula,the second the free Comatula.EXTINCT SPECIES.1. P. Milleri Auxiliary side-arms round, in single series ; column smooth ;columnar joints generally alternately smaller and larger —P. Caput-medusae,Mill. Crin. 56—In Lias.2. P. basalitiformis Auxiliary side-arms round, in single series; columnsmooth ;columnar joints generallyof more uniform thickness—Mill. Crin.62.— Lias.3. P. tuberculatum Auxiliary side-arms round, in single series ; columntuberculated.— Mill. Crin. 64.— Lias.4. P. subangularis.—Auxiliary side-arms round, in a double series. Mill.Crin. 59.—In Lias.