. 



REPRODUCTION OF PHYLACTOLAEMATA 501 



234, d) acts as a pump, introducing water from the tentacle- 

 sheath into the body-cavity, into which it is said by him to open, 

 and so forcing out the polypide. It is probable that many of 

 the forms which have a stiff, unyielding ectocyst possess special 

 arrangements for introducing water in some way into the space 

 bounded by the ectocyst, 1 and so forcing out the polypide. Such, 

 for instance, may be the median pore which occurs beneath the 

 orifice in Microporella (Fig. 241, A, mp), and in certain other cases. 



Reproduction of Phylactolaemata. Sexual reproduction 

 takes place in Cristatella from June to August. The sperma- 

 tozoa are ordinarily produced on the funiculus. The ovaries 

 usually occur on the inner side of the common wall of the 

 colony, not far below the orifice of a polypide. Each ovary 

 matures a single egg, which develops in situ, the free larva leav- 

 ing the colony by the orifice of one of the degenerated polypides. 



A second method of reproduction takes place by means of 

 the statoblasts, which are developed on the funiculus (Fig. 249). 

 According to Yerworn, 2 each statoblast arises from a single cell 

 of the funiculus ; and on this view, the statoblast is, as supposed 

 by the earlier observers, a special kind of winter-egg. Accord- 

 ing to more recent researches, 3 the funiculus consists of a 

 central axis, formed from the ectoderm, and of an outer sheath 

 of mesoderm-cells ; the statoblast is developed from the two kinds 

 of cells of which the funiculus is composed, and is consequently 

 comparable in its mode of origin to an ordinary bud. Its 

 special peculiarities are : its origin as an internal bud, its posses- 

 sion of a chitinous shell, and the fact that it . is destined to 

 leave the parent colony, and to develop, after a period of rest, 

 into a new colony. Germination takes place by the forma- 

 tion of a polypide-bud inside the statoblast, which finally splits 

 along its equator into two halves. The contents emerge as a 

 young colony which possesses at least one fully-formed polypide. 



Eemarkable structures known as " hibernacula " occur in the 

 fresh-water Ctenostomes, Paluclicella and Victorella. These bodies 

 are in the former (Fig. 250, B) specially modified external buds, 

 which persist through the winter when the rest of the colony 

 dies down. At the close of winter the shell splits into two 



\} See P. Cambridge Soc. vol. xi. Part 1, 1901.] 

 8 Zeitschr. wiss. Zool. xlvi. 1888, p. 124. 



3 Kraepelin, Abh. Ver. Hamburg, xii. 1893, No. 2, p. 47 ; Braem, Bibl. Zool. 

 (Bd. ii.) Heft 6, 1890, pp. 6Q f. 



