TYPE PROSOPTGIA. 267 



tissue in the neighborhood of the mouth of the zooecium. The colony 

 resulting from continued budding becomes accordingly as a rule much more 

 compact than in the Endoprocta, each polypide being more or less approxi- 

 mated to its predecessor. 



Closely related to the process of budding is that of regeneration, also 

 of frequent occurrence among the Polyzoa. Among the Endoprocta Pedi- 

 cellina shows the process in a periodical though not simultaneous moult- 

 ing of the polypides, new ones developing from the tip of the stalk which 

 bore the amputated polypide. Here, as in ordinary budding, the tissues 

 concerned appear to be ectoderm and mesoderm, the stalk containing no 

 prolongation of the original endoderm. 



In the Ectoprocta, however, regeneration is carried to a greater extent. 

 In examining any colony of Biujula, for example, in some of the zocecia in 

 addition to the polypide a brown mass may be seen, the so-called " brown 

 body" (Fig. 116, bb) ; in others the brown body may be seen without 

 any distinct polypide. This body is the result of the degeneration of the 

 digestive tract and other organs of the original polypide, only its body- 

 wall or endocyst persisting, from which new organs are developed and the 

 polypide regenerated. The significance of this process is not clear, but it 

 has been suggested that it stands in relation to the process of excretion, 

 the formation of the brown body occurring in forms which do not possess 

 any special excretory organs. It is now known that in the marine Ecto- 

 procta the excretory products are taken up in part by the cells of the 

 stomach and caecal pouch, a fact which seems to harmonize with the sug- 

 gested significance of the brown body. 



The formation of a new polypide from ectoderm and mesoderm appar- 

 ently is a difficult fact to explain on the theory of the germ-layers. It is 

 possible, however, to regard the tissue from which buds arise as undiffer- 

 entiated embryonic tissue passed on from polypide to polypide and trace- 

 able back to the embryonic tissue of the ovum. In the formation of each 

 polypide a certain amount of the tissue becomes differentiated, but some 

 still retains its embryonic character, a continuation of the budding process 

 being thus possible. 



Affinities of the Polyzoa. There seems to be little room for doubt but 

 that the Endoprocta represent more nearly the original Polyzoa than do 

 the Ectoprocta. Their colony formation is of a more simple form than 

 that of the other group, they possess nephridia which are wanting in the 

 majority of the Ectoprocta, and their development is much simpler, the 

 highly modified larva of the mai'ine Ectoprocta having undoubtedly been 

 derived from one approximating in structure that of Pedicellina, Cypho- 

 nautes representing a stage in the evolution. 



Similarities have been traced between the Pedicellina larva and the 

 Annelid Trochophore, and it is not improbable that this may have been the 

 true derivation of the group, in which case the Polyzoa are to be regarded 

 as forms which have never possessed any traces of metamerism, but stand 

 in about the same relationship to the Annelida as do the Rotifera. 



