SECT. ii. POLYSTOMELLA CRISPA. 47 



shelly canals, radiating from the central chamber, 

 ending in conspicuous pores in the external surface 

 of the shell. During this course the canals send small 

 tubes into the chambers on each side of them ; through 

 these the internal segments of the animal can fill the 

 canals with cords of sarcode, and protrude them into 

 the water, whence they are supplied with food. 



The genus Polystomella is distinguished by the high 

 development of the intermediate skeleton and the canal 

 system that maintains it. The Polystomella crispa 

 (fig. 97, E), a beautiful species common on the British 

 coasts and in other temperate seas, has a lenticular 

 form, the -^ to the -% of an inch in diameter. It con- 

 sists of a small number of convolutions winding round 

 the shorter axis of the lens, increasing rather rapidly in 

 breadth, and each one almost entirely enclosing its pre- 

 decessor, so that the shell is exactly alike on both sides, 

 and only the last convolution is to be seen. At the 

 extremities of the axis there is a mass of solid shell- 

 substance, perforated by orifices which are the apertures 

 of a set of straight, parallel canals. In the figure only 

 the last convolution is visible, upon which the convex 

 septal bands are very conspicuous, dividing the surface 

 into well marked segments, upon the exterior edge of each 

 of which there are strong transverse crenulations. The 

 only communication which the chambers have with the 

 exterior, is by means of a variable number of minute 

 orifices near the inner margin of the sagittate partition- 

 plane, close to its junction with the preceding convolu- 

 tion ; a very high microscopic power is required to see 

 them, as well as the minute tubercles with which the 

 surface of the shell is crowded, more especially on the 

 septal bands and in the rows of depressions between the 

 segmental divisions. 



The sarcode animal itself corresponds exactly with 

 the form and spiral arrangement of the chambers so 



