RHABDOPLliURA MlllABlLlS. 31 



haps be considered as the ventral corners of the lophophore, 

 and losing themselves at the root of the tentacular arms. 

 They appear to represent the retractile muscles of the tenta- 

 cular arras, which produce their flexion upwards and back- 

 wards. It has not been possible for me to discover any dis- 

 tinct trace of any such thin membrane connecting the basis 

 of the tentacles (the so-called calyx), as is found in the fresh- 

 water Polyzoa. As regards the tentacles themselves, they 

 are, indeed, of the usual cylindrical form for Polyzoa, and are 

 as usual furnished with cilia, but in the living animal they 

 have a very difl'erent appearance from that of the tentacles in 

 the ordinary Polyzoa. While in the latter they are always in 

 the greater part of their length extended straight, forming a 

 regular corona, which only slightly changes its form, the end of 

 some one or other of the tentacles being only sometimes bent 

 a little in one direction or another, the tentacles in the Rhab- 

 dopleura are always bent and curved in the most irregular 

 manner in all directions (figs. 1 and 2), so that there can 

 be no question of any regular tentacular corona. The num- 

 ber of these tentacles, which, as above mentioned, are at- 

 tached in a double row along the anterior side of the tenta- 

 cular arms, is somewhat various (about forty on each ai-m) ; 

 they are longest about in the middle, and are in this part. 

 about one third of the length of the arm, and diminish some- 

 what towards the base, but more towards the extremity, 

 where they are often quite rudimentary. The tentacular 

 arms themselves, each of which, at the base on the dorsal 

 side, is furnished with a little fascicle of unusually long cilia 

 attached to a small tubercular prominence (figs. 1 and 3, n), 

 are of very considerable length — quite as long as the whole 

 of the rest of the body — of narrow cylindrical form, thickest 

 at the base, and tapering regularly to the end, which is ob- 

 tusely pointed. Anteriorly they are separated (see fig. 4) 

 by a naked, somewhat concave part (extending a little down- 

 wards), here contiguous to the basis of the remarkable oval 

 shield (c), which^ both by its enormous development and 

 peculiar function, below described, forms one of the most 

 peculiar features of the Rhabdopleura. 



Between the bases of the tentacular arms, and from 

 a somewhat ventral point, proceeds in an anterior direc- 

 tion a large and very remarkable prominence, situated 

 longitudinally, which has the form of an oblong thick disc 

 or shield, one surface of which (the dorsal surface) is in the 

 middle grown together with the anterior end of the body, 

 while the ventral surface is free and bordered by a rather 

 thicker raised ridge, distinct from the adjacent parts. The 



