THE PHYLUM ANNELIDA 



plexus layer. The nerves are similar in number and distribution in all the 

 somites, with the exception of those at the anterior end. There is thus a 

 metameric repetition of parts in the nervous system as in other systems. 



Nereis is dioecious, as are most polychaetes. The ovaries and testes are not 

 permanent organs but are formed anew at each breeding season by the differ- 

 entiation of cells lining the coelomic cavities. In some polychaetes these 

 developments are limited to the posterior portions of their bodies. At a 

 specific time in the breeding season, these detach from the anterior ends, swim 

 to the surface of the sea, and congregate in great swarms. Ova and sperma- 

 tozoa are emitted in astronomical numbers during these swarming periods, 

 either through the nephridia or by rupture of the body wall, and fertilization 

 occurs in the sea water. In many polychaetes the zygote develops into a 

 ciliated larva, the trochophore (see Fig. 14.17, p. 418). After a period of 

 free-swimming existence this larva seeks the bottom and transforms into a 

 young adult, with a relatively large head region and only a few so-called 

 "primary" somites. Subsequent growth involves the addition of successive 

 new somites at a region just anterior to the segment bearing the anus (Fig. 

 14.3). This mode of growth, by the addition of "secondary" somites 

 posteriorly, is to some degree characteristic of all annelids. It is interesting, 

 and possibly significant, that in the generalized polychaetes only the 

 secondary somites are capable of producing gonads and gametes. This has 

 given rise to the theory that the long, many-segmented bodies of these 

 worms originally developed as an adaptation for the increase of reproductive 

 capacity. 



Other Polychaetes. In one of the older classifications the polychaetes 

 were divided, on the basis of their mode of life and correlated structural 

 organization, into the Errantia, or wandering forms, which are free-living 

 like Nereis, and the Sedentaria, which are sedentary and live in tubes of 

 somewhat more permanent construction. Although this distinction is rather 

 too artificial to be of value in the taxonomic sense, it is still convenient in a 

 brief consideration of the polychaetes. The worm Amphilrite, which is an ex- 

 ample of the more or less sedentary forms, inhabits a mucus-lined tube in 

 sandy or muddy bottoms. Amphitnte feeds upon microscopic organisms and 

 particles of detritus, collected from the water by long, ciliated tentacles. 

 Pectinaria (Fig. 14.4) builds a conical tube of sand grains cemented to- 

 gether by secretions. This tube is buried in the bottom or dragged about as 

 the worm crawls. Chaetoplerus is a large polychaete which excavates a 

 permanent U-shaped burrow, lined by a parchment-like secreted layer and 

 open at both ends to the water. The worm lies at the bend of the U, and by 

 means of several pairs of large, fan-like parapodia maintains a current of 

 water through the tube. The animal depends on this passing stream for 

 oxygen, for the elimination of wastes, and for its food supply. Micro- 

 organisms are entrapped in a sheet of mucus, produced and manipulated 

 by other highly modified parapodia. 



In all of these sedentary forms the parapodia and the appendages of the 



401 



