Genealogical Relationships 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 solitary 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
Fig. 16.—Diagram of Loxosoma Neapolitanum (after Kowalewsky). A single polypide devoid of buds. m, mouth; st, stomach; shs, basal gland of the polypide-stalk.
lost some of their internal organs, the modern Polyzoa have developed pnri 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 new individuality have suffered a degeneration. The prae-oral lobe (epistome, buccal disk) present in all Polyzoa except the most minute and most elaborately colonial forms namely, the Gymnolaema is to be regarded as an ancestral structure which has been lost by the Gymnoltema. The horse-shoe-shaped lophophore, such as we see it in Phoronis and in Lophopus, 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 zocecia were not at once produced, but possibly dwellings of another kind (Pterobranchia). We aro thus led to look upon the Gymnolyerna as the extreme modification of the Polyzoon type. Starting with an organism similar to Phoronis, we may supposo the following branchings in the pedigree to have occurred.
Vermiformia
A. The complete hippocrepian lophophore becomes specialized in the form of ctenidia or gill- plumes; the epistome enlarged. = PTEROBKANCHIA. a. The anti-tentacular region of the body elongated 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 continuity so as to form a colony of a hundred or more individuals (Rhabdo- pleura). 15. 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 (Loxosomn) or remain to form a small colony (Pedicellina). The telescopic introversibility of the lophophore does not advance be yond an iiiitinl stage. The arms of the lophophore grow round so as to embrace the anus. = Sub-class 1 (of the Eupolyzoa) Entoprocta. P>. The complete hippocrepian lophophore remains in its origi nal form, and also the pros -oral epistome, but the telescopic in troversibility of the anterior region of the body is greatly de veloped at the snnie time that the cuticle of the hinder part of the body is increased in thickness and toughness. Bud production, not from a stnlk-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 statoblasts, and thus are enabled to become isolated and to persist in the peculiar conditions of fresh waters. = The 1st order (of Ectoprocta) Phylactolaema. B. The polypides forming relatively larger colonies, and themselves becoming relatively more minute, loso by atrophy the pra-oral epistome ; and simul taneously the arms of the hij>po- 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 zoœcium, 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) Gymnoliema.
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:—
Cœlomata 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.