8. An Account of Surface Ciliation in some Polychsete Worms.

Abstract

SUMMARY. Epidermal cilia are employed in the production of feeding currents and the collection of food in a number of Polychzte families. In certain cases epidermal cilia are not related to this function. The distribution and arrangement of such cilia and the currents they produce are described for members of the families Aphroditidz, Phyllodocids, Syllidae, Hesionida, Spionida, Sabellariidz, Sabellidae, and Serpulids. Absence of such cilia is recorded in members of the Tomopterids, Nereids, Eunicids, GlyceridAElig;, and ArenicolidAElig;, In the Aphroditidze the interrelation of the genera based on the distribution of epidermal cilia is in complete agreement with that based on other morphological data. The significance of such cilia is discussed. are merely vestigial larval organs. Their employment for the removal of detritus or the “incubation” of eggs is thought to be incidental. It is concluded that their importance lies in the production of respiratory currents. In the Spionids Polydora and Scolecolepis it is shown that the adult cilia are persistent larval structures. The initially locomotor cilia of the larva are retained to meet the respiratory requirements of the adult. In Scolecolepis experimental evidence indicates that the cilia are still under the control of the adult worm, though such control is of no apparent value. The phylogenetic origin of cilia is discussed. It is suggested that they evolved, in the first place, as locomotor organs and were under the control of the organism. Subsequent adaptation for other purposes has probably been responsible for the loss of control with resulting automatism. Epidermal cilia are to be associated with ccelomic cilia and circulation and intestinal cilia as features of the primitive Polychzete. Their disappearance is associated with two tendencies. Muscular activity has replaced ciliary activity on several occasions, in the Aphroditidq Chztopteridz, and Sabellidae. The disappearance is closely associated with the evolution of a contractile vascular system and the association with i t of respiratory pigments. The several stages occurring in this process are exemplified by various families of modern PolychAElig;tes.