60 STARFISH GALLERY. 



The presence or absence of this stalk may be taken as the first 

 character of importance in the classification of Echinodermata, 

 which may be divided into two groups : — 



A. Pelmatozoa*, or Echinoderms provided with a stalk through- 

 out life or in the larval stages only. To this group beloug the 

 Crinoidea, and the extinct Blastoidea and Cystidea. 



B. Echinozoa, or Echinoderms without stalks at any time of 

 their existence. To this group belong the Asteroidea, Ophiuroidea, 

 Echinoidea, and Holothurioidea. 



To understand the differences between these various orders it is 

 well to become acquainted with the structure of the parts that 

 form the base of the cup in Pentacrinus or the disk in Antedon. 



The diagrams and preparations in Case 7c show that the primi- 

 tive composition of the disk is a central plate, around which are 

 two sets of five plates each ; five of these lie along the line of the 

 arms or rays, and are radial plates, and five are interradial, or, as 

 they are often called, basal; in Antedon the top of the stem, which 

 is found in the larval stage only, fuses with this central plate, while 

 the basals cease to appear on the surface ; in Echinus, or the Sea- 

 Urchins, the central plate is absorbed, while the bounding-plates 

 retain their primitive relations ; in the Starfishes and Brittle-stars 

 the plates are less conspicuous, and in the Sea-Slugs they disappear 

 altogether. 



Crinoidea. — This Order may be described as stalked, globular, 

 or cup-shaped Echinoderms, in which the oral surface of the calyx 

 or disk looks upwards, and in which five jointed and generally 

 branched rays arise from the central disk. Their joints have 

 jointed pinnules at their sides, and the sucking-feet have the 

 form of tentacles. 



The stalked representatives of this Order are placed on two tables 

 in the corners by Case 7, and are worthy of being particularly 

 noticed for their fine preservation, size, and beauty. The largest 

 specimen of Pentacrinus decorus was taken on a telegraph-wire, to 

 the covering of which the stalk of the Crinoid is still attached. 

 Metacrinus is a lately discovered genus, which appears to be cou- 

 fincd to the eastern seas. 



* From the Greek pelma=& stalk. 



