PTEROBRANCHIA. 



The two marine genera Rhabdopleura and Cephalodiscus are contained 

 in this group ; the former found near the Shetland and Lofoten Islands, 

 and on the Norwegian coast at depths of 90, 200, and 40 fathoms 

 respectively l ; the latter taken by the ' Challenger ' expedition in 200 

 fathoms off the Patagonian coast. 



Rhabdopleura forms indefinitely branching colonies, attached to 

 various foreign objects. The zooids are all connected by a stem, and 

 are contained in a tubarium, or connected system of hollow tubes com- 

 posed of a hyaline material, which is secreted by the buccal shield of the 

 zooids. The axial part of the tubarium, both main axis and branches, 

 is adherent. The tubes of the zooids are more or less erect. The 

 latter originate from the axis laterally, and the axial tube is divided by 

 transverse septa into compartments, one to each zooid and its tube. 

 Cephalodiscus buds, but the buds are detached at a certain stage of growth. 

 Its zooids and their descendants inhabit cavities in a gelatinous branched 

 mass with fimbriated edges probably secreted by the buccal shield as in 

 Rhabdopleura. The head of the zooid bears in Rhabdopleura, on either 

 side a flexible arm, the two edges of which are beset each with a single 

 row of about fifteen ciliated flexible tentacles. Arm and tentacles are 

 alike supported by a mesoblastic skeleton. Cephalodiscus is provided with 

 twelve similar arms, six oh each side, similarly beset with tentacles, and 

 each of its arms terminates in a knob. Between the bases of the arms in 

 both genera alike, and overhanging the mouth is a mobile disc or buccal 

 shield. Its surface is ciliated in Rhabdopleura, and its shape extremely 

 changeable. Not only does it secrete the tubarium, but it is an organ 

 of locomotion by means of which the zooid creeps up its tube. The body 

 in both genera is more or less ovoid, and gives origin on its oral aspect, 

 and somewhat posteriorly to a flexible stalk or stem ("gymnocaulus"). 

 This stem ends freely in Cephalodiscus, and bears at its extremity one 

 or two buds. In Rhabdopleura the stem is branched correspondingly with 

 the tubarium. It contains an axial skeleton, similar to that supporting 

 the arms and tentacles, one surface of which is covered by muscle cells. 

 The part of the stem to which the zooid is immediately attached, and 

 which traverses the zooid tube, retains its soft nature. Contraction of its 

 muscle cells retracts the zooid towards the base of its tube, the stalk itself 

 being thrown into coils simultaneously. When the muscle cells relax, 

 the elasticity of the skeleton probably comes into play, and aids the move- 

 ment of the zooid up the tube. But the ectoderm of the branched part of 

 the stalk, contained in the main axis and the branches of the colony, 



1 R. compacta comes from deep water off the coasts of Antrim (Hincks, Marine Polyzoa, i. p. 

 581, PI. 72, Figs. 8, 8 a, 9). 



