GENERAL SURVEY OF THE CLASS CH&TOPODA. 233 



ing season is probably common here as elsewhere in marine Invertebrates. 

 In the SyllidcG 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 Myrianida a long chain of sexual zooids is formed. In this 

 way a regular alternation of sexual and asexual generations may arise. 



Some Polychseta 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 

 sides of the body, 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. There are a few transparent pelagic forms, e.g. Tomopteris. 



The sedentary forms lead a sluggish life within various kinds of 

 tubes limy, sandy, papery, or gelatinous. They are not nearly 

 related, but possess in common certain adaptive characters, such as the 

 aggregation of gills, cirri, tentacles, and sense organs to the anterior 

 exposed part of the body ; the reduction of the parapodia, often used 

 solely for clambering in the tube; the absence of "jaws," and the 

 habit of feeding on minute Algse or other substances suspended in 

 water. Among these are included Serpula, which forms twisted limy 

 tubes outside shells and other marine objects ; the aberrant Sabellaria> 

 which often builds reefs of porous rock formed of the aggregated sandy 

 tubes ; the common Terebella or Lanice conchilega, with its tubes of 

 glued sand particles ; and the strange phosphorescent Chatopterus^ 

 found in deep water, within its yellow parchment-like tube. 



III. Echiuridee. In holes in the rocks on the warmer coasts of 

 Europe there lives a curious "worm" Bonelliaviridis, of a beautiful 

 green colour, with a globular body, and a long, grooved, anteriorly 

 forked, pre-oral protrusion. Such, at least, is the female ; but the 

 male is microscopic in size, lives in or on his mate, and is exceedingly 

 degenerate. His gut is without mouth and anus, the surface is covered 

 with cilia, and the body cavity almost obliterated. Related to Bonellia, 

 but of less anomalous shape, are a few other forms, like Thalassema 

 and Echiurtis. 



In all, the body in the adult shows mere traces of segmentation; 

 parapodia, cirri, and gills are absent, but, except in the degenerate 

 males, a few setae are always present. The most characteristic structure 

 is the elongated solid proboscis, which has the mouth at its base. The 

 nervous system consists of a gullet-ring and a ventral cord, but the latter 

 is unsegmented, and there is no brain. The gut is coiled, and bears a 

 curious adjacent tube known as the "collateral intestine," and a pair 

 of excretory "anal vesicles," opening from gut to body cavity, and 

 formed in development from nephridia. The anus is terminal, there 

 is a. closed vascular system, and one to four pairs of nephridia. The 

 sexes are separate, the reproductive elements are formed on the walls 

 of the body cavity, and are shed into it. 



