POLYZOA. 709 



by invaginations of the ectoderm. The sucker appears to be the organ 

 by which the larva ultimately fixes itself; and it might, therefore, be 

 considered, at least functionally, as the homologue of the foot-gland of 

 Loxosoma. The larva of Cyclostomata is ciliated and barrel-shaped. At 

 one end is the mouth leading into a stomach. At the other is a pro- 

 minence, the homologue of the ciliated disc inclosed in a cylindrical 

 sheath, the homologue of the corona (?). When a larva fixes itself, 

 its organs, whatever they are, appear to undergo atrophy, and the tentacles, 

 digestive tract, muscles, and nervous system of the adult zooid, make 

 their appearance. The Phylactolaemata also reproduce by statoblasts or 

 winter buds formed from the funiculus. The cells at one pole of the 

 bud grow round the remaining cells, and form at their inner ends the 

 chitinous investment and chitinoid marginal air-cells of the fully formed 

 statoblast. The statoblasts are discharged on the death and decay of the 

 parent, and the whole colony in Pectinatella is at this time set free, and 

 floating along the stream scatters them over a wide area. The same is 

 true of Cristatella. The statoblast remains quiescent through the winter 

 season. In spring it gives origin to a small non-ciliated, but fully formed 

 individual, which fixes itself, and produces a colony by gemmation. All 

 Polyzoa increase by budding, and the bud appears to be formed by 

 tissue elements derived from the corresponding elements of the parent 

 (see p. 236). In all the marine Polyzoa the tentacles, digestive tract, 

 and its retractor muscles, with the nervous system of the individual are 

 lost in the older parts of the colony. They degenerate into the * brown 

 body.' A new set of organs is produced from the endocyst by budding, 

 and the brown body appears to pass into the new stomach where it may 

 break down and perhaps be partially digested. It or its remnants are 

 eventually expelled. 



The Phylactolaemata are confined to fresh-water. The Gymnolaemata 

 with the exception of Paludicella are marine, and are most plentiful near 

 the shore, and at moderate depths. Certain Cheilostomata were, however, 

 dredged at great depths in the voyage of H.M.S. Challenger, e.g. in the 

 North Atlantic at 2750 fathoms, in the North Pacific at 3125. The 

 Cyclostomata and Ckeilostomata occur fossil. The first-named appear 

 in Cambrian and Silurian strata, the second are sparingly represented in 

 the older strata, but become numerous in the Upper Greensand, whilst 

 the Cyclostomata diminish in numbers from Tertiary strata to the present 

 time. Of living genera Stomatopora among Cyclostomata^ and Hippothoa 

 among Cheilostomata are said to appear in Silurian times. Many Jurassic 

 genera still survive. 



The class Polyzoa is subdivisible as follows : 



I. Entoprocta. Lophophore praeoral ; mouth and anus within the lophophoral 



