IX ENTEROPNEUSTA—BODY EPITHELIUM 563 



B. The proboscis is joined to the second division — the collap, by 

 a short stalk or neck. 



This region forms a somewhat projecting ridge anteriorly round 

 the neck of the proboscis, its anterior wall surrounding the neck like 

 a stand-up collar. The neck is only joined to the collar dorsally, for 

 between the neck of the proboscis and the ventral wall of the collar, 

 gapes the wide unarmed mouth. This leads into the buccal cavity, 

 which runs through the collar. The proboscis is thus a preoral 

 division of the body. The collar is marked oft" from the trunk by a 

 circular furrow of varying depth, over which it sometimes bulges out 

 posteriorly. Further, immediately in front of its posterior boundary, 

 there is a circular furrow round the collar itself. 



C. In the long trunk, in the genera Ptychodera and Schizocardimn, 

 three regions can be distinguished : the branchio - genital, the 

 hepatic, and the abdominal regions. 



1. The anterior part of the branchio - genital region, which 

 follows the collar, is distinguished by the branchial pores, and the 

 posterior part by the gonadial apertures. The gonads, however, may 

 stretch for some distance anteriorly into the branchial, and posteriorly 

 into the hepatic regions. The branchial pores are found on the 

 dorsal side, arranged in two longitudinal rows, or, when they are 

 small and circular, in two more or less deep longitudinal furrows 

 converging posteriorly. The pores may take the form of transverse 

 slits. 



The genus Ptychodera is distinguished by a longitudinal fold or 

 ridge on each side for the reception of the gonads. These two 

 genital folds (Fig. 455, A) can, when well developed, bend towai'ds 

 one another over the back, and so form, on the dorsal side of the 

 branchial region, a branchial vestibule. 



2. The hepatic region is distinct only in Ptychodera (Fig. 455, A) 

 and Schizocardiimi. In these genera it is distinguished by two 

 longitudinal rows of projecting brown or green liver-cseca. Even in 

 cases in which these caeca do not appear to be arranged in two 

 longitudinal rows, it can be shown that their insertions on the body 

 form two such rows, but, there not being sufficient room for their 

 swollen ends one behind the other, many of them are pushed out 

 irregularly to the right or left. A mediodorsal strip of the hepatic 

 region always remains uncovered. 



3. The cylindrical, delicate walled abdominal region tapers off 

 posteriorly, as a rule gradually, to the terminal anus. 



II. The Body Epithelium. 



The body is everywhere covered by a thick ciliated epithelium, 

 in which, apart from the nerve-elements, undifterentiated epithelial 

 cells and gland cells can be distinguished. The latter are always 



