OUGO'I'REMA P8AMMITES. 239 



sinuses in the brancliinl niesliwork ; but in Oligotrema^ in 

 which the branchial sac is very small^and the oesophagus and 

 stomach are relatively larger and occupy the posterior two- 

 thirds of the body, the endoplerome is a well-defined, con- 

 tinuous sheet of tissue surrounding the oesophagus and 

 anterior portion of the stomach. As the peribranchial cavities 

 do not meet in the mid-ventral line, the branchial sac, iind in 

 Oligotrema also, the oesophagus and stomach, are attached to 

 the ventral body-wall by a partition consisting of a sheet of 

 plerome covered on both sides by atrial epithelium. This is 

 the ventral or sub-endostylar suspensory fold. 

 There is a similar dorsal suspensory fold separating such 

 prolongations of the right and left peribranchial cavities as 

 extend beyond the limits of the cloaca. The relations of these 

 folds to the ecto- and endoplerome are shown in the diagi'ara 

 in the text. Fig. 3. A justification for this somewhat lengthy 

 discussion of facts already well known will be found, I hope, 

 in the practical utility of the terms I have proposed. I will 

 now give a detailed description of the anatomy and histology 

 of Oligotrema. 



The Test. 



The external appearance of the test has been mentioned 

 in the description of the species. It is relatively thin over 

 the greater part of the body, but is locally thickened in the 

 neighbourhood of the branchial and atrial apertures and on 

 the external aspects of the proximal moieties of the arms. It 

 is involuted to a considerable extent, both at the branchial 

 and atrial apertures. The branchial involution forming the 

 branchial siphon is thick, and its free surface is thrown into 

 a number of longitudinal folds. The atrial involution of the 

 test is thin, but extends for some distance inwards (fig. 34). 

 The substance of the test is homogeneous, or in some places 

 shows traces of fibrillation, and gives the cellulose reaction 

 with sulphuric acid and iodine. It contains numerous immi- 

 grant corpuscles (amoebocytes), which are nowhere swollen up 

 to form the large vesicular cells common in the tests of simple 



