1 52 ZOOLOGY FOR MEDICAL STUDENTS chap. 



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

 portion of the larva, which subsequent development proves to be the 

 anterior end, there is a cushion-like thickening of the ectoderm (s.g) 

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

 cells forming this project into the water as long sensory hairs, looking 

 like stiff cilia and probably to be regarded as cilia which have lost their 

 original motor function and taken on a new sensory one. Between the 

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

 some scattered and others forming a compact band on each side. 

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

 known as protonephridium which instead of having an open nephrostome 

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

 cells " which will be described in the next chapter (p. i6i). 



The trochosphere 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 

 trochosphere becomes the head region. In fact the trochosphere larva 

 might be described as the precociously developed and free-swimming 

 head of the Polygordius ! 



The species of Polygordius which occurs in the North Sea and round 

 the west coast of the British Isles shows a curious peculiarity in its 

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

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

 sphere. 



In the typical Polychaeta the course of development is in general 

 much as in Polygordius. There is typically a trochosphere larva although 

 this may be modified in details such as its shape and more especially 

 the arrangement of its cilia. 



The main characteristics which serve collectively to mark off the 

 Polychaeta as a distinct group of annelids are (i) the mg,rine habit ; 

 (2) the presence of groups of chaetae usually embedded in distinct 

 parapodia ; (3) the well-developed head region usually provided with 

 tentacles, palps, or other projections ; (4) the separate sexes ; and (5) 

 the occurrence of a free-swimming larval stage. 



II. Oligochaeta 



The group Oligochaeta includes the true Earthworms, of which a 

 large number of species and genera are known, and also a number of 



