POLYZOA AND TUNIC ATA 157 



CHAPTER XV. 

 POLYZOA AND TUNICATA. 



547. AT the lower extremity of the great series of Molluscous animals, 

 we find two very remarkable groups, whose mode of life has much in 

 common with Zoophytes, whilst their type of structure is conformable in 

 essential particulars to that of the true Mollusks. These animals are for 

 the most part microscopic in their dimensions; and as some members of 

 both these groups are found on almost every coast, and are most interest- 

 ing objects for anatomical examination, as well as for observation in the 

 living state, a brief general account of them will be here appropriate. 



548. POLYZOA. The group which is known under this name to 

 British naturalists (corresponding with that which by Continental Zoolo- 

 gists is designated Bryozoa) was formerly ranked as an order of Zoophytes; 

 and it has been entirely by Microscopic study that its comparatively high 

 organization has been ascertained. The animals of the Polyozoa, in con- 

 sequence of their universal tendency to multiplication by gemmation, are 

 seldom or never found solitary, but form clusters or colonies of various 

 kinds, and as each is inclosed in either a hornv or a calcareous sheath or 

 * cell/ a composite structure is formed, closely corresponding with the 

 'polypidom' of a Zoophyte, which has been appropriately designated the 

 polyzoary. The individual cells of the polyzoary are sometimes only con- 

 nected with each other by their common relation to a creeping stem or 

 stolon, as in Laguncula (Plate xxn.); but more frequently they bud-forth 

 directly, one from another, and extend themselves in different directions 

 over plane surfaces, as is the case with Flustrce, Lepralice, etc. (Fig. 380); 

 whilst not unfrequently the polyzoary develops itself into an arborescent 

 structure (Fig. 381), which may even present somewhat of the density 

 and massiveness of the Stony Corals. Each individual, designated as a 

 polypide or polype-like animal, is composed externally of a sort of sac, of 

 which the outer or tegumentary layer is either simply membranous, or is 

 horny, or in some instances calcified, so as to form the cell; this investing 

 sac is lined by a more delicate membrane, which closes its orifice, and 

 which then becomes continuous with the wall of the alimentary canal; 

 this lies freely in the visceral sac, floating (as it were) in the liquid which 

 it contains. 



549. The principal features in the structure of this group will be best 

 understood from the examination of a characteristic example, such as the 

 Laguncula repens; which is shown in the state of expansion at A, Plate 

 xxn., and in the state of contraction at B and c. The mouth is sur- 

 rounded by a circle of tubular tentacles, which are clothed by vibratile 

 cilia; these tentacles, in the species we are considering, vary from ten to 

 twelve in number, but in some other instances they are more numerous. 



