POLYZOA. 



626 ] 



POLYZOA. 



Tlie cells are of various forms, mentioned 

 under the genera, and the j are often furnished 

 with bristles or spines. At the orifice of 

 each cell the tentacles and more or less of 

 the body of the animals are protruded. In 

 the marine or Infuudibulate order, the struc- 

 ture of the cell-mouth is used as a charac- 

 ter, — those in which itis terminal and simple 

 (PI. 41. fig. 30) forming the Cyclostomata ; 

 and those in which it is subterminal, curved, 

 and furnished with a movable lid, the 

 Cheilostomata (PI. 41. fig. 5 b) ; whilst in 

 the Ctenostomata there is a comb-like cir- 

 cular fringe of bristles connected by a mem- 

 brane surrounding ths cell-orifice, visible 

 when the body is partly protruded. Most 

 are fixed ; but CridatcUa is free and loco- 

 motive, having a discoid base. 



Curious appendages are found attached 

 mostly to the cells of the polyzoaries. The 

 first are called bird's-head processes or avi- 

 cuJaria (PI. 41. fig. 5h*, and fig. 20). 

 They consist of a body (fig. 26 c/), a hinge- 

 or lower -jaw-like process (fig. 26 e), and a 

 stalk (/). They are attached by the stalk 

 to the interior of a round hollow process, 

 projecting slightly from the surface of the 

 polyzoary (fig. 26 a). The body is divided 

 by an oblique ridge (fig. 26 d) on its inner 

 surface into two chambers. The lower 

 portion is moved up and down by an 

 elevator and depressor muscle (fig. 26 c). 

 During life the motion is constant ; and it 

 continues long after the death of the animal. 

 These bodies appear analogous to the pedi- 

 cellaria of the Echinodermata. 



The second kind, called vihracula, consist 

 of a hollow process (fig. 5 cl, b), from which 

 a vibrating filament (fig. 5 d) projects. 

 The interior of the process is filled witli a 

 contractile substance which moves the 

 filament. 



The body is usually oblong or elongate. 

 At its anterior end is a ring or disk {lupho- 

 phore), upon which the tentacles are placed ; 

 tliis is perfect in the Infundibulata, but 

 deficient at one part, or horse-shoe shaped 

 in the Ilippocrepia (PL 41. figs. 3 c & 9). 

 The tentacles are hollow, closed at the end, 

 uncontractile, coated externally with cilia 

 on the sides next each other, and communi- 

 cate with the cavity of the body through 

 apertures in the disk. In most of the Ilip- 

 pocrepia, the tentacles are surrounded at the 

 base by a transparent cup dike membrane 

 (cali/x), prolonged somewhat upon each ten- 

 tacle, and mostly dentate at the margin. 



Digestive System. — The mouth is situated 



in the middle of the tentacular disk (PI. 41. 

 fig. 3 c), and is closeable in the Ilippocrepia 

 by an epiglottis-like hollow valve {ejnstome), 

 which IS absent in the Infundibulata; at the 

 base of this valve is an aperture which per- 

 forates the disk to open into the abdominal 

 cavity. The mouth terminates in a pharynx 

 (PI. 41. fig. -l e, /), and oesophag-us (tig. 18, 

 b, d), often of considerable length, which is 

 sometimes succeeded by a strongly muscular 

 gizzard. Next comes the stomach (figs. 

 5 e, b, 18 f), often very capacious, and 

 with an appendix (fig. 18 e), and finally the 

 intestine (fig. 18 y), which terminates out- 

 side, but close to the disk (fig. oe,c). Thus 

 the alimentary canal is bent upon itself, the 

 two orifices being very near each other. 



The alimentary canal consists of three 

 coats — an inner rugose, composed of cells 

 with brownish contents, and representing a 

 liver ; a middle, composed of colourless 

 nucleated cells ; and an outer, thin, cellular 

 coat, probably containing muscvUar fibres. 

 The mouth and more or less of the upper 

 portion of the alimentary canal are ciliated. 



The walls of the cavity of the abdomen, 

 the interior of the disk and of the tentacles 

 aU communicate, and are filled with a clear 

 liquid, in which irregular particles float, 

 and in which a constant rotatory motion 

 exists, produced partly by muscular action, 

 and partly by cilia. This liquid corresponds 

 to a chylaqueous fluid, and performs the 

 chyliferous, sanguiferous, and respiratory 

 functions ; for there are no distinct respira- 

 tory organs, nor blood-vessels. 



The muscular system is well developed, 

 the fibres being transversely striated — the 

 principal, or retractors, arising from the 

 bottom of the cells, and being inserted into 

 tlie sides of the oesophagus, so as to exert 

 a retracting action upon the body. There 

 are also parietal muscles, which are in the 

 form of circular bundles running transversely 

 round the cell; they project the polypide. 



The nervous system consists of an oval 

 ganglion placed between the oral and anal 

 apertures, and giving ott' branches to the 

 tentacles, alimentary canal, &c. ; and there 

 is stated to be a nervous connexion between 

 all the cells of a zoarium, called the colonial 

 system. 



Reproduction. — The Polyzoa are propa- 

 gated by gemmation, and by the agency of 

 sexual organs. 



Two kinds of gemmation occur. In the 

 first, the gemm£e are developed externally 

 from the parent cells, and usually near the 



