I RHABDOPLEURA PHORONIDEA 2/ 



tube which becomes embedded in the adherent wall of the outer 

 tube. The thin dark axis, to which the name Rhaldopleura 

 refers, is the feature by which the animal can most readily be 

 recognised without magnification. 



The outer transparent tube is constructed by the proboscis, or 

 buccal shield, the secretion of which appears to be intermittent, 

 so that the tube consists of a series of rings piled on one another. 

 The animal crawls up the inside of its tube by means of its 

 proboscis, while it is retracted by means of the muscles of its 

 stalk. 



The growing axis ends in a row of young buds, the buccal 

 shields of which early reach a relatively large size. The 

 terminal bud gives rise to tube-rings, so that the axis is sur- 

 rounded by a cylindrical outer tube, which becomes interrupted 

 by transverse septa, each bud, except the end one, thus lying in 

 a closed chamber. The wall of each chamber becomes perforated, 

 and the buccal shield then prolongs this perforation by adding 

 tube-rings, the formation of which continues till the tube reaches 

 a considerable length. The bud remains connected with the 

 axis by means of its narrow proximal region, which forms its 

 stalk. The adherent part of the adult colony thus consists of 

 a row of short tubes, traversed by the common axis of the 

 colony. Each tube is produced laterally into the upright tube 

 of an individual. 



The general anatomy closely resembles that of Cephalodiscus. 1 

 There are five body-cavities and a notochord. Collar -pores 

 exist, but proboscis-pores arid gill-slits have not been described. 

 The dorsal region of the collar bears only a single pair of 

 arms. 



Order III. Phoronidea. 



The structure and development of Phoronis (Fig. 13), have 

 already been described in Vol. II. 2 of this series ; and Master- 

 man's investigations, then published in a preliminary form 

 only, are there alluded to. Since then this author has pub- 

 lished fuller accounts 3 of his results, which, if substantiated, 



1 See, however, Conte and Vaney, C. R. Ac. Sci. 135, 1902, pp. 63, 748. 



2 Pp. 450-462. 



3 Quart. J. Micr. Sci. xl. 1898, p. 281 ; xliii. 1900, p. 375 ; xlv. 1902, p. 485. 



