CLASSIFICATIONS IN NATURAL HISTORY. 



In the adult there is developed a more or less resisting integu- 

 ment (15), either in the form of coriaceous (15^) integument, 

 with granules (15^), or spicules (15^), or definitely formed 

 and articulated calcareous plates (i $d). There is elaborated 

 a peculiar hydrostatic apparatus, called the ambulacral water- 

 vascular system (16), which subserves the purposes of locomo- 

 tion (i6#) and the conveyance of food particles (i6) into the 

 mouth, and may be considered as a special elaboration of the 

 elements which are tentacles in the Ccelenterata. In the 

 Echinodermata there is a considerable elaboration of the ali- 

 mentary system. There is a closed gastric cavity '(17), separate 

 from the somatic or vascular cavity (i 8) ; this constitutes a rudi- 

 mentary stomach. In the more perfect type of the Echino- 

 derms, the Echinoids, there is a distinct alimentary canal of 

 several parts, composed of a mouth (19), provided with special 

 organs for reducing food, five teeth (20), and a differentiated 

 cesophagus (21) leading to a stomach (22), and a distinct intes- 

 tine (23) terminating in an anal (24) opening. There is alsa 

 a pulsatory heart (25), with radiatory vessels, or blood-vascular 

 system (26). Thus in this higher type of Echinodermata we 

 find already differentiated organs for mastication, digestion, 

 nutrition, and distribution or circulation. It is not well 

 established that the function of respiration is specialized, or 

 that distinct organs are differentiated for this function. The 

 Starfish (Asterioidea) do not have distinct teeth, but the 

 Ophiuroidea do, and the haemal system is present in both, but 

 the mouth in many cases serves for ejection of fcecal matter. 

 These two types are developed very early as early as the Or- 

 dovician, so that it is evident that all these differentiations of 

 the Echinoderm type of the digestive system were elaborated 

 by the beginning of the; Ordovician, and probably in the Cam- 

 brian. The Crinoids and Cystoids were Cambrian, the Blas- 

 toids appeared later; the digestive functions were less elab- 

 orate in them, but the differentiation into a stomach or 

 digestive cavity, as distinct from the nutritive tract or intes- 

 tines, was present. The nervous system was also developed 

 as a ring about the mouth, or oesophagus, and sent out nerves 

 to the other parts of the body, and there are pigment-cells 

 developed on the upper side of the Echinoids, which are re- 



