ON THE DIPLOCHORDA. 293 



the time a leiii^th of "5 mm. was reached it had greatly elon- 

 gated, and formed a long cylindrical portion behind the tenta- 

 cles. This was confirmed by MetschnikofF (14) and by others. 

 A similar phenomenon is found in Tornaria, according to 

 Morgan (17), the area between the post-oral and the perianal 

 bands being very small in his younger stages, but elongating 

 greatly afterwards. 



It is a very necessary process in Actinotrocha in order 

 that the requisite space for the accommodation of the ventral 

 diverticulum may be forthcoming. 



An inspection of the figs. 18 — 27 inclusive (PI. 20) will suffi- 

 ciently indicate the further relationships of the areas in which 

 the epiblast is thickened to those in which it is attenuated. In 

 fig. 28 is seen the marked contrast in this character of the 

 cells on the dorsal and the ventral surfaces of the pre-oral 

 lobe. In fig. 29 are to be noticed the two oral grooves (o. g.) 

 passing out from the mouth. They are longitudinal depres- 

 sions in the ventral collar area^ and are also found in Cepha- 

 lodiscus. In fig. 26 the right-hand groove is seen entering 

 the side of the mouth. They can be traced on to the general 

 surface of the collar. On the ventro-lateral side of the pre-oral 

 lobe are also two grooves (fig. 24, a. g.), which pass from the 

 antero-lateral corners of the mouth round the edge of the pre- 

 oral hood. Their bounding cells, like the rest of the ventral 

 surface of the hood, are without cilia and much attenuated. 

 They maybe termed the atrial grooves, and no doubt serve for 

 the removal of the water which is brought into the mouth 

 by the action of the ciliated oral grooves. In fact, they are 

 the true analogues of the gill-slits of the Chordata, though 

 perhaps not homologous with these. 



The histology of the several areas is sufficiently indicated in 



a series of transverse sections (PI. 19, figs. 6 — 10, 16, 17, and 



PI. 21, fig. 39). 



Nervous System. 



The nervous system of Actinotrocha has been little 

 studied, and some of the observations made with respect to it 

 are without doubt erroneous. Wageuer (20) describes in some 



