SECTION VII 



PHYLUM TROCHELMINTHES 



THE typical larval form of a number of the groups which have 

 yet to be studied is a form which is known as the Trochosphere or 

 Trochoplwrc. It is necessary that a clear idea should be formed 

 at this stage of this important larva, reference to which will very 

 frequently be made in the sections that follow. The general 

 shape of a typical trochosphere is oval or pear-like (Fig. 244) with 

 a broader and a narrower end and distinct bilateral symmetry. 

 Encircling the body about the middle, or rather nearer the broad 

 than the narrow end, is a double circlet of strong cilia, the prce- 

 oral circlet (pr.or.ci.) vv prototroch, situated on a corresponding ring- 

 like thickening of the ectoderm; 

 behind the mouth is often a second 

 circlet of cilia, the post-oral circlet, 

 and a ciliated groove or ciliated 

 streak usually runs backwards from 

 it along the middle of the ventral 

 surface. The mouth, situated just 

 behind the prasoral circlet, leads into 

 an alimentary canal, which at first 

 runs nearly transversely, and then 

 bends round so as to run back to- 

 wards the narrow end, near which 

 it opens on the exterior in an 

 anal aperture. About the middle 

 of the broader (anterior) end of 



the trochosphere is a thickening, the apical plate (Br), project- 

 ing from which are usually a number of sensory cilia; and in 

 many trochospheres eye-spots and a pair of short tentacles occur 

 in close relation with the apical plate, which is the nerve-centre 

 of the larva. A pair of ciliated tubes which may be present are 

 the excretory organs or nephridia. 



Jii.ci 



FIG. 244. A Trochosphere. >/,>,. ci. uiial 

 cilia ; Br. brain ; cnt. stomach ; 

 Msd 1 . mesodermal bands ; pro. (///<. 

 intestine ; pr. or. ci. prse-oral circlet 

 of cilia ; stdni. gullet. (From 

 Parker's Biology, after Fraipont.) 



