228 PHYLUM ANNELIDA. 
peritoneal membrane beside the blood vessels supplying 
the funnels of the nephridia. The reproductive cells are 
liberated into the body cavity, and there matured. They 
pass out by the nephridia, and may be temporarily stored 
in the bladder portions of all but the first. Little is known 
in regard to the development, beyond the fact that the 
young are for a time free-swimming pelagic forms. 
Development of Polychzta.—As an example of the development 
of the marine Chetopods, we may take Hzfomatus, which has been 
investigated by Hatschek. Here segmentation is complete, but some- 
what unequal, and results in the formation of a blastula, with its upper 
hemisphere composed of small (ectodermic) cells, and the lower of large 
(endodermic) cells. Among these latter are two spherical cells—the 
primitive mesoblasts. Invagination takes place in the usual way to 
form a gastrula; the primitive mesoblasts divide and form mesoblastic 
bands. During these processes the external form has altered con- 
siderably. The apical (aboral) region of the gastrula becomes tilted 
forward, an ectodermic invagination arises posteriorly, and, uniting with 
the archenteron, produces hind-gut and anus, while a similar insinking 
anteriorly, in the region of the blastopore, forms fore-gut and mouth, 
The larval gut so formed has a distinct ventral curve. Cilia appear on 
the surface at an early stage, and now form a distinct pre-oral ring, and 
also a less constant post-oral ring. At the apex of the pre-oral region 
an ectodermic thickening takes place; this gives rise to an apical 
ganglion, with which sensory structures are often associated. The 
mesodermic bands give rise to muscle cells, used in swimming, and also 
to the ‘‘ head kidneys ”—a pair of larval excretory tubes. The larva so 
formed is a typical Trochosphere, such as occurs in the great majority of 
Polycheeta, in a more or less modified guise in many other worm-types, 
and also in Molluscs. Its chief characters are the following :— 
(1) There is a prominent pre-oral region, with an apical ganglion and 
a ring of cilia. 
(2) The gut has a distinct ventral curve, and a threefold origin. 
(3) The larval body cavity is simply the persistent segmentation 
cavity, and in it posteriorly lie the primitive mesoblasts. 
The Trochosphere is a free-swimming pelagic larva, which, among 
worms, corresponds largely to the future head region of the adult. Its 
metamorphosis into the adult probably takes place in the most primitive 
fashion in the little worm folygordius. We shall therefore follow it 
there (Fig. 119). 
In the larva, which is a typical Trochosphere, the first sign of 
segmentation appears in the bands of mesoblast. These become divided 
into successive segments, while at the same time the posterior region of 
the larva elongates greatly, carrying the larval gut backwards with it. 
Meanwhile a cavity appears in each of the mesoblastic segments. 
These cavities, taken together, form the adult body cavity; the outer 
and inner walls form the somatic and splanchnic layers; the posterior 
and anterior walls of adjacent segments fuse to form the septa of the 
adult worm ; the inner (splanchnic) walls of the primitive segments on 
