200 SEGMENTED WORMS OR ANNELIDA. 



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 Polychseta. — As an example of the development 

 of the marine Chastopods, we may take Enpomatus, 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 Polygordius. We shall therefore follow it 

 there (Fig. 89). 



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 

 each side fuse above and below the gut to form the dorsal and ventral 

 supporting mesenteries of the gut. The head region is at first dispro- 



