220 PHYLUM ANNELIDA. 



also occur in other regions of the body. Apart from the parapodia, the 

 shape and appearance of the body are most affected by the condition of 

 the septa. In the active free-living forms (Errantia) these are usually 

 present throughout the body, and give a characteristic worm-like 

 appearance. In burrowing and tubicolous forms (Sedentaria) the septa 

 tend to be suppressed. Their absence facilitates burrowing, by per- 

 mitting free movement of the ccelomic fluid, and is often associated with 

 a division of the body into regions, and a loss of the typical uniform 

 shape (cf. Arenicola). 



With regard to internal organs, the gut is frequently branched and of 

 large calibre. In some cases (Capitellidre) it possesses an accessory 

 communicating tube (Nebendarm), which is of interest, because it has 

 been compared to the notochord of Vertebrates. The nephridia function 

 as genital ducts ; they are often reduced in number, and may, as in the 

 common Lanice conchilega, be united by longitudinal ducts, which 

 have been compared to the segmental ducts uniting the excretory tubes 

 of young Vertebrates. Though the sexes are usually separate, there 

 are a few hermaphrodite forms, and the aberrant Sternaspis, where the 

 reproductive system recalls that of Oligochaetes, is an exception to 

 the rule that the organs are simple. There is a metamorphosis in 

 development, and some interesting peculiarities occur in regard to 

 reproduction. Thus several species of the common genus Nereis, when 

 sexually mature, have the body divided into two regions a posterior 

 region containing the ova or sperms, and an anterior unmodified 

 asexual region. The posterior region is distinguished by the structure 

 of its parapodia, which become converted into broad, flattened 

 swimming organs, and there is sexual dimorphism. Worms of this 

 peculiar type were long described as a distinct genus under the name 

 of " Heteronereis," and even yet the subject is imperfectly understood, 

 for there is from unknown causes much variation as regards the extent 

 of the modification. A complete change of habit at the spawning 

 season is probably common here as elsewhere in marine Invertebrates. 

 In the Syllidae a phenomenon occurs similar to the formation of a 

 " Heteronereis," but a process of fission may result in the division of 

 the modified form into an anterior asexual zooid and a posterior 

 sexual one. In this way a regular alternation of sexual and asexual 

 generations may arise. 



The Polychaeta were formerly classified as active and sedentary 

 forms, but few are permanently active, and the classification is now 

 abandoned. It is, however, necessary to realise that while certain 

 forms dwell habitually within tubes, others are at least at times active 

 and free-living. The latter have usually well-developed parapodia and 

 sense organs, the anterior part of the gut may be furnished with strong 

 jaws, the body is more or less uniform, and the worms are carnivorous. 

 These forms are all included in the sub-order Nereidiformia, which 

 embraces such familiar animals as the common sea-mouse (Aphrodite], 

 with its mass of iridescent bristles covering the dorsal surface, the 

 species of Nereis and Nephthys, so common under stones on the shore, 

 and others equally remarkable for beauty of colour. The bright colours 

 may be due to the iridescent cuticle or to pigments. 



The sedentary forms lead a sluggish life within various kinds of 



