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, more usually, trochophore. 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 trochophore is oval or pear-like 

 (Fig. 268) 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 pre-oral circlet 

 (Wkr.) or prototroch, 

 situated on a corre- 

 sponding ring-like 

 thickening of the ecto- 

 derm ; behind the 

 mouth is often a 

 second circlet of cilia, 

 the p o s t-o r a I circlet 

 (wkr.), and a ciliated 

 groove or c i li a t e d 

 streak usually runs 

 backwards from it 



WAr 



Neph ' 

 Msbr 



FIG. 268. Trochophore. A. 



_.. . ^. anus ; d.LM. dorsal 



muscles ; ED. rectum ; J. stomach ; /i intestine ; 

 Mstr. rnesoderm-band ; n. nerves ; Neph. nephridia ; 

 O. mouth ; Oe. oesophagus ; ozLM. ossophageal longi- 

 tudinal muscle ; SP. apical plate ; v.LM. ventral muscle; 



'./yJV. lateral nerve; Wkr, wkr., pre- and post-oral ^^.^^..,^1^^, j. j. v .LLL j. v 

 I >ands of cilia; WS. apical cilia ; M-Z. adoral cilia. (From ] j-V,. ;jJ1 rt 4.1, 

 Hertwig's Zoology, after Hatschek.) along the middle OI the 



ventral surface. The 



mouth, situated just behind the pre-oral circlet, leads into an 

 alimentary canal, which at first runs nearly transversely, and then 

 bends round so as to extend back towards the narrow end, near 

 which it opens on the exterior by an anal aperture. About the 

 middle of the broader (anterior) end of the trochophore is a thicken- 

 ing, the apical plate (SP.), projecting from which are usually a 



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