MOSS-ANIMALS. 



423 



sea-mat (Flustra foliacea). 

 a, Stock (nat. size) ; b, Five cells (magnified). 



form branched leaf -like lobes very common at the seaside, and often mistaken for 



seaweed. Each side of the leaf consists of a layer of closely crowded individuals. 



The cells only partly calcify, so that when 



fresh they are elastic, and the whole stock 



remains very flexible. To this suborder 



belong also Retepora and Lepralia, above 



mentioned. 



An important rise in the scale of 

 organisation is found in the Gymnolaemata, 

 especially in the lip-mouthed forms, where 

 a marked division of labour takes place; 

 that is to say, the individuals which con- 

 stitute the stock vary in structure and 

 fulfil different physiological functions. 

 There are structures known as zooecia, 

 stolons, avicularia, vibracula, and ovicells, 

 some — perhaps all — of which are modified 

 individuals. The zooscia are the normal 

 individuals of the colony, fully developed for most of the functions of life ; respira- 

 tion, taking in food, and digestion, and no doubt also for receiving sensory impres- 

 sions. The stolons have a much humbler function, but are indispensable for the 

 well-being of the colony. They are rootlike outgrowths of the stock, consisting of 

 very simple individuals which serve for attaching the whole colony to foreign 

 objects, such as stones, shells, etc. The most remarkable are the structures known 

 as avicularia, so called because they resemble the head of a bird. The individual 

 is turned into a pair of forceps, of which the large upper blade (very like the skull 

 and upper jaw of a bird) and the smaller lower blade (like the lower jaw) con- 

 stantly open and shut by means of a complicated arrangement of muscles. These 

 avicularia are movably attached by a short neck, and are found near the entrance 

 to a zooecium. They turn from side to side, snapping in all directions, and, no 

 doubt, every now and then catch some of the small worms, crabs, or larvae which 

 rest on the colony. The victims are held till they decay, and, as they break down, 

 fragments are drawn into the mouth by the water currents caused by the cilia on 

 the tentacles. These dead creatures act as baits, and attract other victims within 

 the influence of the same stream. These also are drawn into the mouth. On 

 account of these peculiar structures, the Chilostomata have been called the bird's- 

 head corallines. Equally interesting again are the vibracula, long thread-like 

 structures, attached by short stalks, which keep up a constant whip-like motion. 

 Their function is not clear ; but perhaps they may be specialised tactile organs, or 

 may help to drive the minute prey within reach of the nutritive individuals. 

 Lastly, we have the ovicells, or egg-receptacles, which are found at the lower ends 

 of the zooecia in the form of bells, helmets, or vesicles. It is uncertain whether 

 these are independent modified individuals or merely appendages of the zooecia, 

 the latter view being the more probable. 



Round-Mouthed Another suborder of the Gymnolaemata consists of the Cyclostomata 

 Group. or round-mouthed Bryozoans. In the tube-like forms (Tubulipora), 



