630 PEOFESSOR B. EAY LANKESTEE. 



from its ab-oral face, near the limit between it and the thorax. 

 It is to the oral face of the abdomen that the gymnocaulus is 

 attached, or rather it is at this spot that the polypide narrows 

 to the form of a stalk. Diverging right and left of the body 

 from the thorax are the two branchial arms, or lophophor-arms 

 as they are best termed in view of the fact that they are the 

 same organs as the arms of the hippocrepian lophophor of 

 other Polyzoa (PL XXXVIII, figs. 1, 2, Ga.). These carry 

 each two rows of cib'ated filaments, usually fifteen in a row, 

 though I have sometimes observed a few more or less. On the 

 aboral surface of each lophophor arm, at its junction with the 

 thorax, Sars discovered a ciliated tubercle (PI. XXXVIII, figs. 

 2, 3, K.), which is possibly related to the osphradium of Mol- 

 lusca, since that sense-organ occupies a similar position in 

 relation to the ctenidium ; and it is not improbable, whatever 

 view may be taken of the relations of other regions of the 

 body of Rhabdopleura to the regions and body-lobes of Mol- 

 lusca, that the ctenidia of the latter are the genetic equivalents 

 of (homogenetic or homologous with) the lophophoral arms of 

 Polyzoa. 



Pigment. — The epidermic cells which clothe these various 

 regions of the body of Rhabdopleura develop at intervals black 

 and orange-brown pigment, so as to give a spotted leopard-like 

 appearance to the animal, which is represented for the first 

 time in the coloured drawings accompanying this memoir. 

 The epidermic cells are either colourless, entirely brown, or 

 entirely black (see PI. XLI, fig. 1). They have a very defi- 

 nite arrangement, and the coloured cells are especially abun- 

 dant on the thorax, buccal-disc, and lophophoral arms and 

 tentacles. The coloured cells are deficient on the abdomen, 

 but are more or less abundant on the contractile gymnocaulus. 



An accumulation of fine spherical pigment corpuscles at the 

 superior dorsal margin of the buccal-disc must be regarded as a 

 rudimentary special sense organ for the perception of light 

 (see PI. XXXVIII). 



Alimentary Canal and Coslom. — The sac-like abdomen 

 is very closely fitted by the wide stomach and reflected intes- 



