1 g Echinoderinata. 



upper edges or between the second radials. Thaumatocrinus has an anal appen- 

 dage supporting the posterior part of the disk as in some Ichthyocrinidae and 

 Reteocrinidae. The arms proper begin with the first joints above the primary 

 radials ; though for purposes of description the successive ray divisions are called 

 radials, distichals, and palmars etc., and the term brachials is limited 

 to joints beyond the last axillary. The supposed syzygies in the arms of Bathy- 

 crinus are really trifascial articulations. Hyocrinus has unusually large pinnules, 

 almost equal in length to the parts of the arms above each pinnule bearing joint, 

 very much as in the Palaeozoic Barycrinus. The pinnules of Actinocrinidae are 

 homologous with those of Neocrinoids and not with the ambulacral plates of Cyatho- 

 crinidae as supposed by Wachsmuth & Springer. For ambulacral plates are 

 fully developed in both Pentacrinidae and Comatulidae, being best seen in the 

 pinnules. There are two rows of side plates on each of which rests a moveable 

 covering plate (Saumplattchen , Miiller). The Bourgueticrinidae have covering 

 plates only. The interradial areas of the disk are often very closely paved with 

 anambulacral plates which extend out on to the arms and are sometimes greatly 

 developed on the genital pinnules. In Bathycrinus and Rhizocrinus there are large 

 interradial diverticula of the gut supported by winglike processes from the skeletal 

 joints below. The gut of Actinometra makes four coils round the disc instead of 

 only one as in the endocyclic Crinoids. - - Ludwig's description of the vascular 

 system is very generally confirmed. Hyocrinus and Holopus have one or more of 

 the oral plates pierced by water-pores, which may occur on both arms and pin- 

 nules, always opening into the genital canal. The branching tubules depending 

 from the oral blood-vascular ring open into a dense labial plexus which is con- 

 nected with the ventral branches of the ovoid or plexiform gland, with the genital 

 vessels of the rays, and with the intervisceral vessels. In Promachocrinus and in 

 Antedon Eschrichti and its allies the part of this labial plexus between mouth 

 and anus is much modified and forms an eggshaped spongy organic which consists 

 almost entirely of a delicate reticulum of connective tissue. The plexiform gland 

 is connected below with the cavities of the chambered organ which are continued 

 downwards into the vascular axis of the stem, and in the Pentacrinidae form a 

 miniature chambered organ at every node, each chamber giving off a cirrus-vessel. 

 Ludwig's account of the genital glands is confirmed and extended, the chief 

 addition being the occasional presence of ovaries in the disc as well as in the pin- 

 nules. The nervous sheath surrounding the vascular axis of the stem sends 

 numerous branches into the substance of the stem joints which probably unite 

 into a subepidermic plexus. Similar branches are given off from the axial cords 

 of rays and arms towards both dorsal and ventral sides. The latter form a 

 complex parambulacral network in the ventral perisome which sends nerves into 

 the spinelets on the disc, when present. The function of the Sacculi is still un- 

 explained. Abundant in most species of Antedon, they are rare in the other 

 endocyclic Crinoids and never occur in Actinometra. Pentacrinin occurs in the 

 Atlantic species of Pentacrinus as well as in those from the Pacific, and also occurs 

 in Holopus. - - The relationship of the Neocrinoids to the Palaeocrinoids is dis- 

 cussed at length, with especial reference to the vault of the latter group and its 

 homologies with the calyx-plates of the abactinal side. The general results are 

 expressed in the following scheme which shows the mutual homologies of the 

 principal plates in the actinal and abactinal systems of Echinodermata. 



