420 MORGAN. [Vol. V. 



The apical plate itself is seen in cross-section, in Fig. 45. 

 The ectoderm is here exceedingly thick, and has beneath it a 

 broad zone of nerve-fibre layer. To the lower end of the plate 

 the muscle band is attached, and runs thence to the anterior en- 

 terocoel. The apical plate must form the larval nervous system 

 to a very large extent, if we may judge by the thickness of the 

 nerve-fibre layer. The plate differs, however, very essentially, I 

 think, from the apical plate of the Trochophore, inasmuch as 

 there are no nerve-fibre bands running from it. This can be 

 determined by staining the apical plate and examining the inner 

 surface. The ectoderm and nerve-fibre layer are simply contin- 

 uous with the general extra-oral area. 



Scattered mesenchyme cells are found in the blastoccel space 

 between the digestive tract and ectoderm of Tornaria. The 

 blastoccel space itself is filled by a watery somewhat gelatinous 

 substance, and scattered in it here and there may be found a 

 few mesenchyme cells as shown in Figs. 33, 34, 35, 40. Some- 

 times one or two may be seen in such sections applied to the 

 ectoderm, both over the circum- and extra-oral areas, but these 

 are not numerous, and do not seem to form a continuous plexus. 

 Portions of the ectoderm cut away, stained, and mounted with 

 the inner surface upwards, will generally show a few of these 

 amoeboid-like cells. Also now and then a mesenchyme cell can 

 be seen on the wall of the stomach or intestine. But the most 

 interesting fact is that these cells are relatively very numerous 

 over the oesophageal wall as shown in Fig. 33. Over the wall 

 they form almost a continuous layer of branching cells. The 

 interesting bearing of this will be seen in a later comparison 

 between Tornaria and the Echinoderm larvae. 



Metamorphosis of Tornaria. 



Agassiz has given most excellent figures of the larva during 

 its period of metamorphosis. The larva becomes more opaque, 

 and changes its shape, becoming more elongated antero-posteri- 

 orly. The whole of the anterior region of the digestive tract 

 becomes pulled inwards, so that the gill-pouches lie at a more 

 posterior level of the embryo — behind the lower limb of the 

 longitudinal ciliated band. The anterior enteroccel does not 

 take part in this movement, and consequently comes to lie 



