70 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



carried away by the ciliary currents. In order to do this, the pinnule tip has been 

 greatly contracted aborally. 



Between the usual distal pinnule segments the transversely striated muscle 

 bundles are very small, almost rudimentary, and the greater part of the facets of the 

 segments is occupied by the ligamentary connections. This structure does not permit 

 more than rather weak and slow contraction. Between the pinnule segments in the 

 region of the combs, on the other hand, the transversely striated muscle bundles, 

 especially those of the aboral part, are enormously developed, which allows of strong 

 and rapid contraction. Gislen believes that the aboral muscle, by stronger growth 

 gradually forced the weaker aboral muscle toward the side and thus secured a more 

 medioventral position for itself. Owing to this the pinnule tip was no longer able to 

 curve in laterally when contracted, but was rolled up ventrally, and the comb must, 

 in order not to lose its effect, proceed up the aboral side of the pinnule. Gisle'n said 

 that in this way we may explain the greatly one-sided strengthening of the muscles, 

 the torsion of the pinnule tip (twisted at right angles to the longitudinal direction), 

 and the progress of the dorsal spines up the lateral side turned from the mouth. 



In the Comasteridae, as pointed out by Gisle'n, branches from the dorsal nerve 

 run to the bases of the teeth, and these may possibly be of some importance in con- 

 nection with the senses of touch or of taste. As a consequence of the rich innervation 

 the pinnule tip should be able to determine whether an object seized is edible or 

 inedible. It is often difficult to draw a sharp line between the ciliary currents for 

 cleansing purposes and those carrying food. Here, too, the possibility of an alterna- 

 tion of function may easily be presumed. When the combs have once begun to 

 grasp small particles that have fallen upon the animal, it may easily be imagined that 

 instead of letting the object drop down between the arms they may put it into the 

 ambulacral groove or into the mouth. From this it is not a very long step to begin 

 picking up or pinching off small particles algal threads, bits of bryozoans, pieces of 

 leaves, etc. which do not fall directly down upon the animal, but which are found 

 hi its immediate neighborhood. In this way, according to Gisle'n, the formation of 

 the combs upon the oral pinnules may be explained. 



The dorsal hooks on the distal pinnules were used as anchoring organs. These 

 would sometimes catch hold of some soft loose object from which small pieces could 

 be severed. By curving in the tip of the pinnule over the ambulacral groove of the 

 arm or of the pinnule itself the combs were able to pass on the particles of food to the 

 ciliary current to be transported to the mouth. In this way a torsion must also have 

 been developed in the formation of the pinnule, owing to the one sided development 

 of the muscles. The combs upon the distal pinnules might have arisen hi this way. 

 The animal by means of these structures was supplied with an extra (and more heavily 

 digested) contribution of food, the intestine therefore becoming lengthened and the 

 ambulacral grooves reduced as being unnecessary. 



In certain comasterids, as hi the species of Comissia, Comatilia, and some forms 

 of Comactinia echinoptera, the combs are limited to one or two pairs of the proximal 

 pinnules. These forms are endocyclic, except possibly one form of Comactinia 

 echinoptera. Gisle'n believes that hi these and other endocyclic comasterids the 

 combs are of predominating importance for cleansing and anchoring purposes. 



