COMPOUND NATURE OF STAR-FISHES. 1 65 



Eeliinoderma is limited, without exception, to five. Every 

 Star-fish consists of a central, small, body-disc, all round 

 the circumference of which are attached five or several 

 long articulated arms. Each arm of the Star-fish essentially 

 corresponds in its organization with an articulated ivorm 

 of the class of Ring-worms, or Annehda (p. 149). I therefore 

 consider the Star-fish as a genuine stock or cormiis of 

 five or viore articulated worms, which have arisen by the 

 star-wise growth of a number of buds out of a central 

 mother-worm. The connected members, thus grouped like 

 the rays of a star, have inherited from the mother-worm 

 the common opening of the mouth, and the common diges- 

 tive cavity (stomach) lying in the central body-disc. The 

 end by which they have grown together, and which fuses 

 in the common central disc, probably corresponds to the 

 posterior end of the original independent worms. 



In exactly the same way several individuals of certain 

 kinds of wonns are united so as to form a star-like cormus. 

 This is the case in the BotrylUdw, compound Ascidians, 

 belonging to the class of the Tunicata. Here also the pos- 

 terior ends of the individual worms have grown toecethei', 

 and have formed a common ovitlet for discharges, a central 

 cloaca ; whereas at the anterior end each worm still pos- 

 sesses its o^vn mouth. In Star-fishes the original mouths 

 have probably become closed in the course of the historical 

 development of the cormus, or colony, whereas the cloaca 

 has developed into a common mouth for the whole cormus. 



Hence the Star-fishes would be compound stocks of 

 worms which, by the radial formation of buds, have 

 developed out of true articidated worms, or Annelids. This 

 hypothesis is most strongly supported by the comparative 



