GENERAL SURVEY OF THE CLASS CH‘ETOPODA. 233 
ing season is probably common here as elsewhere in marine Invertebrates. 
In the Syllidee 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 Myrianidaa long chain of sexual zooids is formed. In this 
way a regular alternation of sexual and asexual generations may arise. 
Some Polychzeta 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 Merezs 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. Zomopteris. 
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 Algze or other substances suspended in 
water. Among these are included SerZz/a, which forms twisted limy 
tubes outside shells and other marine objects ; the aberrant Sade//aria, 
which often builds reefs of porous rock formed of the aggregated sandy 
tubes; the common Zeredella or Lanice conchilega, with its tubes of 
glued sand particles; and the strange phosphorescent Chetopterus, 
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 ”—Bonellia viridis, 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 Bonediza, 
but of less anomalous shape, are a few other forms, like Zhalassema 
and Echiurus. 
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 setee 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 » 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. 
