496 POLYZOA AND COMPOUND TUNICATA. 



ganglion; as yet no branches have been distinctly seen to be 

 connected with it in this species ; but its character is less doubt- 

 ful in some other Polyzoa. 



328. If we scrutinize the foregoing characters, we shall find 

 that the most important of them are Molluscan, rather than Zoo- 

 phytic. In the first place, all true Polypes use their tentacula to 

 grasp their food and convey it to the mouth ; and these tentacula 

 are destitute of cilia; whilst, on the other hand, in all the Ace- 

 phalous Mollusca, the nutritive matter is drawn in by a ciliary 

 current, which also serves to aerate the fluids. Xow the latter, 

 as we have just seen, is the case with the Polyzoa ; and thus, 

 although their arms very commonly present a circular disposition 

 around the mouth, they may be considered as representing, in 

 their relation to the economy of the animal, the ciliated branchial 

 sac of the Ascidians {§ 331). But they do not by any means con- 

 stantly present this radial symmetry ; thus, in the Plumatella, a 

 beautiful fresh-water genus of Polyzoa, the ciliated arms are set 

 upon two lobes or projections, one on either side of the mouth. 

 The structure of the alimentary canal, again, removes the Polyzoa 

 from the zoophytic series. In no true polype is there a separate 

 intestine and anal orifice, nor does the whole apparatus hang 

 freely in the visceral cavity ; and the existence of a gizzard-like 

 organ, and of a rudimentary liver (closely resembling that found 

 in the lowest Tunicata), are also characters of elevation. The 

 most important of all the single characters furnished by the ana- 

 tomy of these animals, is their nervous system ; which, as already 

 pointed out, is distinctly Alolluscan in its type. The absence of 

 a heart and a distinct circulating system is, it is true, a Zoophytic 

 character ; but we shall presently find that even in the Tunicata, 

 which are true Mollusks, the character of the circulating appa- 

 ratus is extremely degraded. The propagation by gemmation, 

 although formerly supposed to be a character exclusively Zoo- 

 phytic, is known to belong also to the greater part of the " tuni- 

 cated" Mollusks; and from this, therefore, no argument can be 

 drawn in favor of the zoophytic nature of the Polyzoa. And 

 although many of their composite fabrics have a stony density, 

 and closely resemble the solid polypidoms of the helianthoid and 

 asteroid Polypes, yet in others, especially amongst the fresh- 

 water species, we find a very close resemblance to the gelatinous 

 bed or leathery crust in which the Compound Ascidians are 

 lodged ; and if we imagine calcareous matter to be deposited in 

 this bed or crust, wo should have a fabric closely resembling that 

 of many stony polyzoaries. 



329. Of all the Polyzoa of our own coasts, the Flustrce or " sea- 

 mats" are the most common; these present flat expanded sur- 

 faces, resembling in form those of many sea-weeds (for which 

 they are often mistaken), but exhibiting, when viewed, even with 

 a low magnifying power, a most beautiful network, which at 

 once indicates their real character. The cells are arrane-ed on 



