314 A. T. MASTERMAN. 



PI, 22, figs. 50 and 51). This is followed by a post-oral segment, 

 which in Actinotrocha is produced into tentacles, and in 

 Balanoglossns is not. This at first sight is a marked dif- 

 ference, but the fact that the collar region in Cephalo- 

 discus is produced into tentacles, and also that in some 

 Tornarise tentacular processes are found post-orally as well 

 as pre-orally, must be taken into consideration, and the secon- 

 darily acquired adult habitat of Balanoglossus renders an 

 atrophy of tentacular processes at least a plausible hypothesis. 

 Behind this is the third portion of the body or " trunk," an 

 elongated cylindrical segment with a terminal (or sub- 

 terminal) anus. 



These segments are, in each type, covered with a unicellular 

 layer of epiblast or ectoderm, which is for the most part 

 ciliated and glandular. 



Nervous System. — The nervous system is in both cases 

 of a primitive type. The inner ends of the outer layer cells 

 form a fine diffused network of fibres underlying the whole 

 surface, but special concentrations can be distinguished in 

 various parts. 



In both Actinotrocha and Balanoglossus (PI. 22, figs. 

 46,47) the chief nervous concentrations are found in the dorsal 

 region of the collar, and form a thick nervous mass extending 

 throughout this region. The differences are of degree only, 

 for in Actinotrocha the dorsal collar region is narrowed 

 from before backwards, and with it the nervous area, and the 

 more or less hollow tubular condition in Balanoglossus is 

 seen in a more nascent condition in the neuropore of Actino- 

 trocha (cf. figs. 48, 49). In each there extend forwards 

 from the main ganglion, on to the pre-oral lobe, three main 

 nerve-tracts, one median dorsal (in the condition of three 

 secondary groups in Actinotrocha) and two lateral bands, 

 skirting the lateral edges of the pre-oral segment and meeting 

 in front. 



Posteriorly the collar nervous area gives off a pair of lateral 

 branches which encircle the extreme hind edge of the collar, 

 with a nervous ring, and from the mid-ventral point of this 



