EXTERNAL STRUCTURE 



453 



crown surrounding the mouth on all sides but one, where there 

 is a slight break in its continuity. The crown of tentacles or 

 lophophore is flattened, and the two ends 

 drawn out, and each is coiled into a spiral 

 (Fig. 229) ; between the bases of these two 

 spirals three ridges can be seen, each ending 

 in a pore ; the median opening is the anus, 

 the two lateral are the openings of the 

 nephridia or kidneys, which also serve as 

 ducts for the reproductive organs. The 

 anus is thus approximated to the mouth, 

 and since the continuity of the tentacular 

 crown is broken at a spot just between 

 the two, there would be nothing to sepa- 

 rate these orifices if it were not for the 

 presence of the epistome, a projection or 

 flap of the body -wall which overhangs 

 the mouth between it and part of the 

 crown of tentacles. 



The extent to which the ridge bearing 

 the tentacles is incurved at each side varies 

 in different species. In Ph. kowalevskii 

 and Ph. psammophila the ends are only 

 slightly turned in, so that the crown of 

 tentacles is truly horse-shoe shaped ; but Fro. 228. A specimen of Ph. 



D , , j . , , j . , busldi M'Int. removed 



in Ph. oMstralis they are turned in and 



from its tube and seen 

 from behind, x about 2. 

 (After M'Intosh.) 



form three coils on each side. The num- 

 ber of tentacles also varies, being very 

 numerous in Ph. australis and Ph. luskii the latter having as 

 many as 300, whilst the other species as a rule have from 60 

 to 90. The bases of the tentacles are fused for a short distance 

 with one another, forming a thin membrane. 



The rest of each tentacle is free, and its inner surface, or that 

 turned towards the mouth, is covered with long cilia, which, by the 

 currents they set up, doubtless serve to bring food to the mouth. 

 The tentacles are hollow, and their cavity is kept open by a stiffen- 

 ing of the tissue, which almost resembles an internal skeleton ; the 

 cavity communicates with the anterior part of the general body- 

 cavity, and up it runs a single blood-vessel containing red blood. 

 A single nerve is also distributed to each tentacle. 



