2oS SEGMENTED WORMS OR ANNELIDA. 



guise in many otlier worm-types, and also in Molluscs. We may 

 summarise the chief characters of the Trochosphere thus : — 



(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 Polys^ordius. We shall, therefore, follow it 

 there (Fig. 68). 



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 W'ith 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 sup- 

 porting mesenteries of the gut. The head region is at first dispropor- 

 tionately large, but later by an independent process of growth becomes 

 reduced. The larva abandons its pelagic life and becomes adult. 



Comparing the development of Polychteta with this, we find that the 

 Trochosphere is often modified, and that segmentation tends constantly 

 to appear at an earlier stage. As a further step in the same direction, 

 we may note that in some Polycheeta the Trochosphere stage is no longer 

 recognisable as such. 



A general co7itrast of the modes of Development in different 



Annelids. 



A. B. 



" Larval " Types " Fretal " Types 



as in as in 



marine Ch.etopods, Earthworm, Leech, &c. 



Polygordiits^ ^c. 



Development indirect. Development direct, within egg 



A free swimming Trochosphere capsule ; Trochosphere stage almost 

 stage, with trimk almost or wholly o'- wholly suppressed, 

 suppressed, with head region 



greatly developed, with adapta- . > ^ 



tions to free marine life. Liiinbiicus type defsiue type 



with little nutri- with much nutri- 



( I live material in live material in 



ovum, with gas- ovum, with gas- 



, trula formed by trula therefore 



' invagination (em- formed by over- 



holic). growth (epibolic.) 



