490 



POLYZOA 



as to bring the calyx into a supine position, with its loDhophore 

 directed upwards. This habit, together with its flattened form, 

 prevents it from being crushed between the worm and its tube. 

 But without some further provision its position might be merely 

 a source of danger. For supposing the calyx to be directed 

 backwards in relation to the worm, a sudden backward move- 

 ment of the latter into its tube might bring the Loxosoma into 

 fatal contact with the inner surface of the tube. There would 

 obviously not be sufficient room to 

 turn round in a vertical plane, so as to 

 bring the body into a position of 

 safety, i.e. into a position in which it 

 moves stalk first. But by a beautiful 

 arrangement of the muscles of its stalk 

 this movement is effected in a hori- 

 -, ft zontal plane ; on touching the Loxosoma 

 with the point of a needle it would 

 swing round in this way through 180 

 with " une rapidite qui etonne." 



Urnatella l is a beautiful form with 

 a segmented stalk, the stalks usually 

 arising in pairs from a common base. 

 It has at present only been found in 

 fresh water in the United States. 



In Pedicellina the plane of the 

 ture of Loxosoma, seen from lophophore is at right angles to the 



ftTSTwS"! stalk > which is se P arated from its ^ 



excretory organ;/, foot-gland; by a marked constriction. In Loxo- 

 ** the lophophore is set obliquely, 2 

 and there is no constriction at the base 

 of the calyx. In Urnatella we find an 

 intermediate condition, the lophophore resembling that of Loxo- 

 soma, while the constriction at the base of the calyx is similar 

 to that of Pedicellina. Since the latter is known to pass in its 

 development 3 through a stage with an oblique lophophore, it 

 may be presumed that Loxosoma is a more archaic form than 

 Pedicellina. In other respects, the structure of the Entoprocta 

 is very constant, whatever the genus. 



1 For structure, see Davenport, Bull. Mus. Harvard, xxiv. 1893, p. 1. 

 a Xo<fc, oblique ; ffwfia, body. 3 Quart. J. Mitr. Sci. xxvii. 1887, pi. xxi. Fig. 10. 



FIG. 245. Diagram of the struc- 



oe, oesophagus ; s, stomach 

 t. retracted tentacles. 



