500 POLYZOA AND COMPOUND TUNICATA. 



Vorticella (Fig. 196). Among the Hippocrepia may be noticed, 

 as exceptional forms, the Cristatella, whose polyzoary is unat- 

 tached, so as to be capable of moving freely through the water, 

 and the Fredericella, the lophophore of which is rather circular 

 than crescentic, the prolongation being so slight as only to be 

 discernible on a careful examination. Generally speaking, the 

 cells are lodged in a sort of gelatinous substratum, which spreads 

 over the leaves of aquatic plants, sometimes forming masses of 

 considerable size. As the animals of this group altogether re- 

 semble the true Zoophytes in their habits, and are found in the 

 same localities, it is not requisite to add anything to what has 

 already been said ( 305) respecting the collection, examination, 

 and mounting, of this very interesting class of objects. 1 



331. Compound Tunicata. The Tunicated Mollusca are so 

 named from the enclosure of their bodies in a "tunic" which is 

 sometimes leathery or even cartilaginous in its texture, and 

 which very commonly includes calcareous spicules, whose forms 

 are often very beautiful. They present a strong resemblance to 

 the Polyzoa, not merely in their general plan of conformation, 

 but also in their tendency to produce composite structures by 

 gemmation ; they are differentiated from them, however, by the 

 absence of the ciliated tentacula, which form so conspicuous a 

 feature in the external aspect of the Polyzoa, by the presence of 

 a distinct circulating apparatus, and by their peculiar respiratory 

 apparatus, which may be regarded as a dilatation of their pha- 

 rynx. In their habits, too, they are more inactive, exhibiting 

 scarcely anything comparable to those rapid movements of 

 expansion and retraction, which it is so interesting to watch 

 among the Polyzoa ; whilst, with the exception of the Salpidce 

 and other floating species which are chiefly found in seas warmer 

 than those that surround our coast, they are rooted to one spot 

 during all but the earliest period of their lives. The larger 

 forms of the Ascidian group, which constitutes the bulk of the 

 class, are always solitary ; either not propagating by gemmation 

 at all, or, if this process does take place, the gemmae being 

 detached before they have advanced far in their development. 

 Since these cannot be considered as Microscopic objects (although 

 no part of their organization can be properly made out without 

 the assistance of that instrument), our attention will be confined 

 to those " Compound Ascidians," the small size and transpa- 

 rency of whose bodies, when detached from the mass in which 

 they are imbedded, not only enables their structure to be clearly 



1 For a more detailed account of the Structure and Classification of this group, see 

 Prof. Allman's "Report on the Fresh-water Polyzoa" in the "Transactions of the British 

 Association" for 1850; Prof. Van Beneden's " Recherches sur les Bryozoaires de la Cote 

 d'Ostende," in "Mem. de 1'Acad. Roy. de Bruxelles," torn, xvii ; Mr. G. Busk's "Cata- 

 logue of the Marine Polyoza in the Collection of the British Museum ;" and Dr. G. John- 

 ston's "History of British Zoophytes." 



