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, troclwphure,. 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. 263) with a broader and a naiTower end and distinct bilateral 



v.LM 

 oe.LM . 



WAr 



d.LM 



Nsph ' 

 Msbr' 



Fig. 263 — Trochopliore. A. anus ; d. IM. dorsal muscles ; BD. rectum ; /. stomach ; ./i. in- 

 testine ; Msir. inesodei-m-band ; «. nerves ; iVep/i. nephridia ; 0. mouth ; Ot. cesophagus ; 

 tfLM. oesophageal longitudinal muscle; SP. apical plate; v.LM. ventral muscle; r.LN. 

 lateral nerve ; Wkr, ickr,, pre- and post-oral bands of cilia ; WS. apical cilia ; wz. adoral cilia. 



- (Fi-om Hertwig's Zoology, after Hatschek.) 



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 ectoderm ; behind the mouth 

 i^ often a second circlet of cilia, the piost-oral circlet (wkr.) and a 

 ciliated groove or ciliated streak usually runs backwards from it 

 Hilong the middle of the ventral surface. The mouth, situated just 



W 322 



