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. 263) with a broader and a narrower end and distinct bilateral 



v.LM 

 oe.LM 



SP 

 v.LN 



O 

 Oe 



-wz 



wkr 



d.LM 



n 



Neph 

 Msbr 



J 



J. 



FIG. 263. Trochophore. A. amis ; d. IM. dorsal muscles ; ED. rectum ; /. stomach ; J l . in- 

 testine ; Mstr. mesoderm-band ; n. nerves ; A'tph. nephridia ; 0. mouth ; Oe. oesophagus ; 

 ceLM. cesophageal longitudinal muscle; SP. apical plate; v.LM. ventral muscle; v.LN. 

 lateral nerve ; W'Av, ickr., pr'e- and post-oral bands of cilia ; WS. apical cilia ; icz. adoral cilia. 

 (From 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 

 is often a second circlet of cilia, the post-oral circlet (wkr.) and a 

 ciliated groove or ciliated streak usually runs backwards from it 

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



322 



