290 A. T. MASTERMAN. 



earlier stages of the larger larvae ; but this cannot be the case, 

 for the structure of the notochord, of the nervous and circu- 

 latory systems, besides the number of tentacles, show clearly 

 that they are at a more advanced stage. In the absence of 

 direct observation of transition stages, there are only two 

 alternative conclusions. These smaller larvae are either later 

 stages, reduced in size as above suggested, or they belong to a 

 different species. Wilson (22) observed two distinct species 

 of Actinotrocha, which he called A and B, differing mainly 

 in the fact that the form A had a shorter intestine and stouter 

 body. 



The whole body of Actinotrocha has three natural divi- 

 sions : 



1. The pre-oral hood (syn. pre-oral lobe, cephalic lobe, 

 protomere), whose position is essentially pre-oral, and from 

 Metschnikoff's figures (14) of early stages it is primarily 

 so. This hood is homologous in every particular with the 

 '' proboscis ^' of Balanoglossus, the "buccal disc'^ (buccal 

 shield) of Cephalodiscus, and the "epistome" of Rhabdo- 

 pleura. 



This pre-oral lobe, whose shape and relationships are shown 

 in the figures already referred to, is produced backwards into 

 two lateral horns (cf. Balanoglossus), which fuse on to the 

 body-wall, laterally to the mouth-opening. 



2. The lophophoral area, or the part bearing the arms. It 

 commences at the hind end of the pre-oral hood, the line of 

 junction being marked by a radial nerve (see Nervous System), 

 and extends backwards a very short way in the dorsal region, 

 but a greater distance in the ventral. It is limited posteriorly 

 by the line of tentacles, or rather by the nerve-ring lying 

 posteriorly to them ; and as this passes diagonally from the 

 front dorsal region (nerve-ganglion) to the ventral side (see 

 PL 18, fig. 3) it is easy to see that this lophophoral segment 

 consists of a cylindrical area surrounding the part immediately 

 behind the mouth, the ventral length of the cylinder being 

 many times longer than the dorsal. This segment or area is, 

 I hope to show, homologous with the " collar" of Balano- 



