P O L Y Z O A 



169 



Oemalogical Eelationships of the Groups of Polyzoa. 



It is necessary that we should try to form some opinion 

 as to which of the various groups of Polyzoa are most like 

 the ancestral form from which they have all sprung, and 

 what are the probable lines of descent within the group. 

 Any attempt of the kind is speculative, but it is absolutely 

 needful since zoology has become a science — that is to say, 

 an investigation of causes and not merely a record of 

 unexplained observations — to enter upon such questions. 

 Colonial organisms have necessarily descended from soli- 

 tary ancestors, and it is probable that the ancestral form 

 of Polyzoa was not only solitary, as are Phoronis and 

 Loxosoma at the present day, but of relatively large size 

 and more elaborately organized than the majority of living 

 Polyzoa. Whilst the polypides have dwindled in size and 



Fio. 16. — Diagram of Loxosoma NeapoUtanum (after Kowalewsky). A single 

 polypide devoid of buds, m, moutli ; st, stomacli ; shs, basal gland of the 

 poiypide-stallc. 



lost some of their internal organs, the modern Polyzoa 

 have developed pari passu with this degeneration an 

 elaborate system of bud-production and colony-formation. 

 The new individuality (the tertiary aggregate) attains a 

 high degree of development (Cristatella, Kinetoskias) in 

 proportion as the constituent units merged in this Hew 

 individuality have suffered a degeneration. The prse-oral 

 lobe (epistome, buccal disk) present in all Polyzoa except 

 the most minute and most elaborately colonial forms — 

 namely, the Gymnolsema — is to be regarded as an ancestral 

 structure which has been lost by the Gymnolsema. The 

 horse-shoe-shaped lophophore, such as we see it in Phoronis 

 and in Lopbopus, is probably the ancestral form, and has 

 given rise to the two other extreme forms of lophophore, — 

 namely, the "pterobranchiate," associated with a great 

 development of the epistome, and the " circular," associated 

 with a complete suppression of the epistome. The ento- 

 proctous lophophore is a special modification of the horse- 

 shoe-shaped, as shown in the diagram fig. 15, C. The 

 formation of zocecia, and so of an elaborate colonial 



skeleton, was not a primary feature of the Polyzoa. Even 

 after budding and colony-formation had been established 

 zooecia were not at once produced, but possibly dwellings 

 of another kind (Pterobranchia). We are thus led to look 

 upon the Gymnolaema as the extreme modification of the 

 Polyzoon type. Starting with an organism similar to 

 Phoronis, we may suppose the following branchings in the 

 pedigree to have occurred. 



Vbrmffohmia 



A. The complete hippocrepian 

 lophophore becomes specialized 

 in the form of ctenidia or gill- 

 plumes ; the epistome enlarged. 



= Pteeobkanghia. 

 «. The anti-tentacular region of 

 , the body elongate,d as a stalk 

 gives rise to one or two 

 rapidly detached buds (Ce- 

 phalodiscus). 

 /3. The stalk gives rise to buds 

 which do not detach them- 

 selves, but remain in con- 

 tinuity so as to form a 

 colony of a hundred or 

 more individuals (Rhabdo- 

 pleura). 



B. The complete hippocrepian 

 lophophore retains its form, but 

 acquires a gradually increasing 

 power of being telescoped into 

 the hinder part of the body. 

 = The Pro-Eupolyzoon. 



A. The anti-tentacular region 

 of the body becomes stalk-like, 

 and develops buds which either 

 detach themselves as they form 

 (Loxosoma) or remain to form a 

 small colony (Pedicellina). The 

 telescopic introversibility of the 

 lophophore does not advance be- 

 yond an initial stage. The arms 

 of the lophophore grow round so 

 as to embrace the anus. 

 = Sub-class 1 (of the, Eupoly- 

 zoa) Entoprocta. 



B. The complete hippocrepian 

 lophophore remains in its origi- 

 nal form, and also the prse-onil 

 epistome, but the telescopic in- 

 troversibility of the anterior 

 region of the body is greatly de- 

 veloped at the same time that 

 the cuticle of the hinder part of 

 the body is increased in thickness 

 and toughness. Bud production, 

 not from a stalk-like pedicle, but 

 from all parts of the body, now 

 becomes characteristic, the buds, 

 which were at first deciduous, 

 now remaining in permanent 

 continuity so as to form colonies. 

 =-The Pro-Ectoprocton. 



A. The polypides acquire the 

 property of carrying their young 

 so as to avoid the disastrous 

 influences of fluviatile currents, 

 and also the property of produc- 

 ing resistent sfatoblasts, and 

 thus are enabled, to become 

 isolated and to persist in the 

 peculiar conditions of fresh 

 waters. 



= The 1st order (of Ectoprocta) 

 Phylactolsema. 



B._ The polypides forming 

 relatively larger colonies, and 

 themselves becoming relatively 

 more minute, lose by atrophy the 

 prse-oral epistome ; and simul- 

 taneously the arms of the hippo- 

 crepian lophophore dwindle, and 

 a simple circum-oral circlet of 

 tentacles is the result. The 

 cuticle of the hinder part of the 

 polypide becomes more and more 

 specialized as the cell or zooe- 

 cium, and in different polypides 

 in various parts of the colony 

 acquires special forms — as egg- 

 cases, snappers (avicularia), ten- 

 tacles, stalk and root segments. 

 = The 2d order (of Ectoprocta) 

 Gymnolsema. 



Distinctive Characters of the Polyzoa. 



From all that has preceded it appears that the really 

 distinctive characters common to all the Polyzoa may be 

 summed up as follows : — 



Ccelomata with closely approximated mouth and anus, 

 the bulk of the body forming a more or less elongate 

 growth at right angles to the original (ancestral) oro-anal 

 axis, and starting from the original ventral {i.e., oral) sur- 

 face. A variously modified group of ciliated tentacles is 

 disposed around the mouth, being essentially the develop- 

 ment by digitiform upgrowth of a post-oral ciliated band. 



