272 



NATURAL HISTORY. 



protrusion takes place at the upper part of the cell, which generally remains soft and flexible, and 

 can often be drawn in, forming a sheath for the tentacles when the alimentary portion of the animal 

 is retracted within the lower and harder portions of the true cell, and can be again everted on their 



re-appearance. The so-called orifice of the cell (Fig. 20, or) 

 is in some instances protected by a row of spines a tac- 

 tile horny sheath crowned with bristles and is in others 

 effectually closed by a movable lip or shutter controlled by 



muscles specially developed 

 for the purpose (Fig. 24, A). 

 The shape of the animal 

 cell is exceedingly variable. 

 Sometimes the exterior is 

 quite plain, or it may be 

 rugged, spinose, or elegantly 

 sculptured, as in the sea- 

 scurfs. In others it is punc- 

 tured with minute pores, 

 which permit of the entrance 

 Fig. 19. A, ALCYONIDIUM GELATiNosuM. (After Van Beneden.) NATURAL SIZE ; COAST o f the sea- water The larger 



OF BELGIUM. B, PLUMATELLA ALLMANI. BROMLEY LOUGH, NOKTHUMBEhLAND. 



(Enlarged, afler Hancock.) of these are Occasionally pro- 



tected by delicate teeth like 



processes, or a fine calcareous network, which acts as a sieve, and guards against the introduction 

 into the interior of the cell of foreign substances, which might prove 

 injurious or inconvenient to the occupant thereof. (Hincks.) Portions 

 of the intervening outer cell-walls are occasionally of thinner structure, 

 and contain minute perforations, through which the soft contents of the 

 various cells are conveyed. These " communication plates " permit of 

 that slow interchange of vital fluid which, with the exception of the 

 association in a common lodgment, is the sole connection now believed 

 to exist between the different members of the same colony. 



Within this double- walled sac, or animal cell, the alimentary organs of 

 the Moss-animal are suspended in the perivisceral cavity (Fig. 20, d), and 

 float freely in a colourless fluid, consisting partly of water admitted from the 

 exterior, and products of digestion which have exuded from the alimentary 

 canal, forming the equivalent of the nutritive fluid of the mollusc. The 

 simple unarmed mouth (m), lying at the base of the tentacular hollow, 

 communicates with a gullet (02), spacious stomach (s), and a long intestinal 

 canal (i), partially lined with cilia. This, bending somewhat abruptly at 

 the base, turns upwards, and terminates in an efferent orifice (a) close 

 to the mouth, occasionally within, but usually without, the circlet of gill- 

 tentacles. The biliary glands are attached to the inner walls of the 

 stomach. A single nerve ganglion (<?), situated between the mouth (m) 

 and the anus (a), with filaments radiating towards the tentacles and in 

 other directions, represents the simple nervous system of the individual 

 polypide. Both reproductive organs are generally present in the same 

 individual. These consist of an ovary (o) attached either to the inner or Flg> 2 * T?***"' ANATOMY 



\ / Or A MOSS- ANIMAL. 



body wall or mantle lining of the cell or above the spermary or testis (Greatly enlarged. After Busk.) 

 (x) situated at the base of the ' perivisceral cavity (d), and connected by 



I _ 11-1 i /,!(! \ -,i .* i i ,, 



an elastic cord-like membrane (the iumculus, z) with the basal walls 

 of the stomach 



The muscles of the Bryozoon, composed of the simplest form of striated 

 fibre, are numerous and well developed. Two pairs of retractors (r), arising from the bottom of the 

 inner lining of the cell, are attached to the alimentary tube, and serve to retract the whole with its 



-d 



(Esophagus: . Stomach: i. Intes- 



une:^ovyy; j.TertMt ,Fanicu- 



lus; or, Onflceof Cfllrar. ousCell; r, 



Ectoeyst; /', Tentacular sin-nth: <*, 

 I'erivisctraf Cavity ; r, Rttract..r 



