(LOGY I-OR MEDICAL STUDENTS CHAP. 



of which tin- larva swims. At tin- apex of the dome-shaped pre-oral 



the lar\a. which subsequent development proves to be the 



rid, thm- is a cushion-like thickening of the ectoderm (s.g) 



the nerve centre or brain of the larva. Certain of the 



nm- this project into the water as long sensory hairs,, looking 



ilia and probably to be regarded as cilia which have lost their 



tor function and taken on a new sensory one. Between the 



derm and ectoderm there is a wide space containing mesoderm cells, 



iitercd and others forming a compact band on each side. 



also present on each side a nephridium (n) of the primitive type 



known as protonephridium which instead of having an open nephrostome 



! inner end is provided with the peculiar structures known as " flame- 



" which will be described in the next chapter (p. 161). 



rot h.isphere becomes converted into the fully formed adult in 



the way indicated by Figs. 72, B and C, the region round the anus growing 



out rapidly to form the body of the worm, while the main mass of the 



,< -sphere becomes the head region. In fact the trochosphere larva 



might be described as the precociously developed and free-swimming 



of tin- Poly-ordius ! 



ipecies of rolygordius which occurs in the North Sea and round 



the \\est coast of the British Isles shows a curious peculiarity in its 



lopmcnt in that the trunk portion of the worm remains for a 



time tolded up in concertina fashion within the body of the trocho- 



gphere. 



In the typical I'olyi -hai-ta the course of development is in general 



much a ^ in rolygordiits. There is typically a trochosphere larva although 



r nav be modified in details such as its shape and more especially 



rraiv.'cment of its cilia. 



nain characteristics which serve collectively to mark off the 



a distinct group of annelids are (i) the marine habit; 



)<-e of groups of chaetac usually embedded in distinct 



the well-developed head region usually provided with 



other projections; (4) the separate sexes; and (5) 



imming larval stage. 



II. Ol.K.nciIAKTA 



ochaeta includes the true Earthworms, of which a 

 numl)( ' r <nd genera are known, and also a number of 





