1885.] NATURAL SCIENCES OF PHILADELPHIA. 301 



or temporarily by the middle of the aboral surface. A jointed 

 stem containing a neurovascular axis is usually present, but may 

 be lost when maturity is reached ; or in the case of a few sessile 

 forms, remain altogether undeveloped. The apical system con- 

 sists of a dorsocentral plate, basals and radials, with the frequent 

 addition of underbasals and interradials. These plates form a 

 cup, which either simply supports or more or less completely 

 encloses the visceral mass, and often bears jointed appendages, 

 the arms and pinnules. 



An oral system, to some extent a repetition of the plates in the 

 apical system, consisting of basals, radials and interradials, 

 covers the peristome, but may be altogether resorbed, or be re- 

 stricted to basals only. The anus either is located within the 

 calyx, and surrounded by abactinal plates, or forms a part of 

 the oral surface. 1 



The water vascular ring does not communicate directly with 

 the exterior, and the lateral branches of the radial vessels (when 

 present) are respiratory, but not locomotor in function. 



Subclass, Crinoidea. 

 Definition. — Pelmatozoa, in which the radial plates of the 

 calyx bear more or less branching arms. These consist of 

 segments which are articulated by means of muscles and liga- 

 ments, and in most cases bear similar jointed appendages, the 

 pinnules. The nervous sj^stem consists (1) of a central organ 

 situated in the calyx, and fibres extending from it through the 

 skeleton of the stem, arms and pinnules ; (2) of a circumoral 

 ring and radial extensions which are in close relation with the 

 ciliated epithelium of the ambulacral grooves. These are more or 

 less extensively distributed on the ventral surface of the disk, 

 arms and pinnules ; and are bordered by groups of tentacles 

 which alternate on opposite sides. When they are absent, the 

 radial water vessels give off no tentacular branches. The water 

 vascular ring opens by five or more water tubes into the body 

 cavity, which itself communicates with the exterior by a corres- 



1 Carpenter's version: "An oral system, consisting of a central plate 

 (orocentral) and five orals, is developed above the peristome of the larva 

 to a very variable extent, and may be either altogether resorbed, or reacb 

 a high degree of importance by the appearance of additional plates so as to 

 form a vault or tegmen calycis. The anus is situated on the oral surface, 

 which may be bare, or more or less covered by calcareous plates. ' ' 



