COMPOUND NATURE OF STAR-FISHES. 165 
-Echinoderma is limited, without exception, to five. Every 
Star-fish consists of a central, small, body-dise, all round 
the circumference of which are attached five or several 
long articulated arms. Hach arm of the Star-fish essentially 
corresponds in its organization with an articulated worm 
of the class of Ring-worms, or Annelida (p. 149). I therefore 
consider the Star-fish as a genuine stock or cormus of 
five or more 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-dise. 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 worms are united so as to form a star-like cormus. 
This is the case in the Botryllide, compound Ascidians, 
belonging to the class of the Tunicata. Here also the pos- 
terior ends of the individual worms have grown together, 
and have formed a common outlet for discharges, a central 
cloaca; whereas at the anterior end each worm still pos- 
sesses its own 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 articulated worms, or Annelids. This 
hypothesis is most strongly supported by the comparative 
