56 Cicmings — Development of some Paleozoic Bryozoa. 



Fig. 25 has been modified from Barrois' original figure in 

 accordance with his later views. 4 The differences in the 

 development of the Cyclostomata and Chilostomata consist 

 mainly in the small size of the adhesive organ in the former, 

 and the great size of the sucker invagination, together with the 

 vestigial condition of the retractile disc and absence of the 

 pyriform organ. The cylindrical shape of the Cyclostome larva 

 is paralleled among the Chilostomata by Serialaria (fig. 33). 



Classification of the Growth Stages of the Colony. 



The phylastic stage has already been alluded to. This is the 

 earliest stage capable of fossilization. It represents the period 

 from the first appearance of a hard exoskeleton to the comple- 

 tion of the protoecium. In the Bryozoa, as pointed out by 

 Barrois, 1 the exotheca at first differs considerably in appearance 

 and texture from the more compact investment of later stages. 

 The buds of the next generation (primary buds) originate at 

 or slightly before the close of this stage. The protoecium 

 frequently resembles the later formed zooecia of ancestral tvpes 

 (Nitsche* Pergens 28 ). 



The nepiastic stage is taken to represent the period from the 

 formation of the primary buds, i. <?., those buds which are given 

 off by the protoecium, to the establishment of the definite bud- 

 ding habit of the colony. The researches of Nitsche, 26 Per- 

 gens, 28 and others, have shown that the early budding habit 

 frequently differs considerably from the later or normal habit 

 of the colony, and more nearly resembles that of ancestral types. 

 This stage may be conveniently subdivided into ana-, meta- and 

 paranepiastic substages. The first, or ana-stage, will include 

 the protoecium and completed primary buds. The meta-stage 

 marks the termination of encrusting growth in forms that pro- 

 duce erect zoaria. The initial circle of buds in Fenestella 

 belongs to this stage. The para-stage comprises the transition 

 from the latter to the neanastic stage. 



Figs. 36-43 show the primary buds of several types of 

 recent Bryozoa. It will be noticed that in every case the first 

 formed buds are lateral, and that a median bud may or may 

 not be present. It is also noteworthy that only one of these 

 eight types has more than three primary buds. The Cyphonautes, 

 fig. 38, has six, an altogether exceptional nuiriber.* 



* The order of budding is indicated by the small Arabic numerals, and the 

 generation of a given bud by the Roman numerals. 1, indicates the median 

 bud ; 2, the left lateral ; and 3, the right lateral bud. In the case of buds of 

 succeeding generations the position number of each preceding generation is 

 affixed to each bud, so that its entire ancestry is thereby expressed. For exam- 

 ple, in fig. 39, bud No. IVs3 32 is of the fourth generation and was derived 

 from the right lateral primary bud through the left lateral of the second gen- 

 eration, and the right lateral of the third generation. Where the same position 



