TYPE ANNELIDA. 247 



APPENDIX TO THE TYPE ANNELIDA. 



Class PnoBONiDiE. 



The class Plioroniclse includes a single genus, Phoronis, of 

 wMch but a few species are known. They are all marine 

 forms of comparatively small size, reaching in some cases a 

 length of 50 mm. Each individual is contained within a 

 chitinous tube to which particles of sand are in some cases 

 agglutinated, and is worm-like and cylindrical in form, the 

 anterior extremity of the body being provided with a horse- 

 shoe-shaped fold, termed the lopJiophore (Fig. 112, a), bearing 

 a number of tentacles arranged around its margins. Between 

 the two circles of tentacles is situated the mouth (b), over 

 which hangs a fold known as the epistome, representing the 

 prostomium or prseoral lobe of the larva. Outside the area 

 enclosed by the tentacles is the anus, on either side of which 

 a pore, the opening of a nephridium, is found. 



The ectoderm of the body-wall is separated by a distinct 

 basement-membrane from a layer of circular muscles, within 

 which is a second layer of longitudinal muscles (i) — an arrange- 

 ment resembling that found in the body-wall of the Annelids. 

 Internally the longitudinal muscle-layer is lined by a layer 

 of peritoneal cells enclosing a spacious ccelom. Near the 

 anterior end of the body there is a transverse septum sepa- 

 rating off, more or less perfectly, an anterior chamber, with 

 which the cavity of the epistome and of the lophophore com- 

 municates, from a larger posterior chamber in which lie the 

 intestine and reproductive organs, and which is divided lon- 

 gitudinally by three mesenteries extending from the intestine 

 to the body-wall. One of these mesenteries accompanies the 

 intestine throughout its entire extent, while the other two 

 lateral mesenteries are in connection only with the sides of 

 the descending limb of the intestine. 



The tentacles are processes of the body-wall, with a 

 ciliated ectoderm, and contain a chitinous axial supporting 

 tissue. 



A completely closed blood vascular system is present, 

 consisting below the transverse septum of two longitudinal 



