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Briefly describe Cephalodiscus and Rhabdopleura. 



(The chief differences between these forms and the Balanoglossida 

 are printed in italics). 



They belong to the Enteropneusta. Their general struc- 

 ture is similar to that of the Balanoglossida ; but they 

 present several features of difference here indicated. 



Cep_halodJ,scus_ forms, colonies. The individuals or zooids 

 are of minute size (about 2 mm. in length), and each one 

 lives in a tube of flexible brown substance like chitin, which 

 is secreted by the proboscis. The zooids are not attached 

 inside these tubes, nor are they connected one with another ; 

 but the tubes are closely massed together. The size of a 

 colony may be about 9 by 6 inches. 



The body is greatly reduced lengthways, but broadened 

 dor so-ventr ally. The stout trunk has, on the ventral sur- 

 face, a stalk-like projection, the pedicle, at the end of which 

 new zooids are formed by budding (asexual reproduction). 

 The " collar " has projections, 4 to 6 pairs of arms bearing 

 numerous tentacles. The proboscis is broad and disc-shaped 

 (buccal disc). The gut is bent like the letter U, so that the 

 anus is dorsal and almost level with and near the ventral 

 mouth. There is only one pair of gill-slits and one pair of 

 gonads. The sexes are usually separate (sexual repro- 

 duction), and development is direct. Budding (asexual 

 reproduction) is frequent. 



Cephalodiscus is marine ; and has been found at various 

 epths (Straits of Magellan, Antarctic, Malay Archipelago, 

 and in other seas). 

 ~ A Rhabdopleura forms colonies. The zooids are of minute 

 yU size, and each one lives in a tube of clear chitin-like sub- 

 I stance made (in ring-like sections) by the proboscis. The 

 basis of the colony is a branching and creeping stolon, 

 containing a chain of living matter from each link of which 

 a zooid arises more or less erect. The zooids are therefore 

 connected by a common organic axis ; but they are not 

 closely massed together. 



The body is borne on a narrow stalk, which is the ventral 

 pedicle of the short stout trunk. The " collar " has one 

 pair of arms with tentacles. In the stalk, arms and tentacles 

 there is a supporting skeletal tissue. By contracting the 

 muscular stalk, the zooid withdraws down the tube ; it 

 moves up again by means of the proboscis. The proboscis 

 is a buccal disc. The gut has the same conformation as that 

 of Cephalodiscus. There are no gill-slits ; but there are 

 two branchial grooves. Rhabdopleura is marine ; it is 

 found at moderate depths, and is widely distributed. 



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