MOLLUSCOIDA. 171 



ciliated head from the aperture, and of again retiring within 

 it, if alarmed. The skeleton is, in some cases, furnished with 

 curious organs, which are known as " bird's-head processes " 

 (Fig. 76, 3), from their resemblance to the beak of a bird. 

 The parts of this beak keep constantly snapping together, 

 very much like the little pincer-like organs called " pedicella- 

 rias " in the sea-urchins and star-fishes ; but it is difficult to see 

 what service they perform. They continue their movements 

 long after the death of the polypides, and this appears, in some 

 cases, at any rate, to be due to a peculiar system of nerves 

 known as the " colonial " nervous system. In addition, namely, 

 to the single ganglion with which each polypide is furnished, it 

 has been shown that in many forms the zoftids composing the 

 colony are united together by a well-developed nervous system, 

 and are thus brought into organic connection with one another. 



The vast majority of the Polyzoa are fixed, and thus as- 

 sume a very plant-like appearance. There is one fresh-water 

 species, however (viz., Cristatella), in which the colony can 

 creep about upon a flattened base very like the foot of a slug. 

 In this same form, also, alone of all the Polyzoa, there is not 

 any outer covering or ectocyst to the polypides. 



The Polyzoa are partly inhabitants of the sea and partly 

 of fresh water, and they are thus divided into two groups 

 which differ from one another very much in anatomical struc- 

 ture. In most of the fresh-water Polyzoa the tentacles are 

 borne upon a crescentic disk or stage (Fig. 77, 3), so that the 

 crown of tentacles assumes the shape of a horseshoe. In 

 almost all the marine forms, on the other hand, the tentacles 

 (Fig. 77, 2) are simply arranged in a circle. 



All the Polyzoa are hermaphrodite, each polypide being 

 furnished with the reproductive organs proper to the two 

 sexes. The eggs are simply liberated into the body-cavity, 

 where they are fertilized ; but it is uncertain how the fertilized 

 ova escape into the external medium. Besides true sexual 

 reproduction, and besides the power of producing colonies by 

 continuous budding, fresh individuals can be produced in many 

 cases by a process of discontinuous gemmation. 



CLASS II. TUNIC ATA (Lat. tunica, a cloak). The mem- 

 bers of this class are not uncommonly called Ascidian Mol- 

 lusks (Gr. askos, a wine-skin) from the resemblance which 

 many of them exhibit in shape to a two-necked leather bottle 

 (Fig. 78, 2). They are popularly known as "Sea-squirts," 

 from their power of .forcibly ejecting water from the orifices 



