298 ZOOLOGY OF THE IXVERTEBUATA. 



the inter-canal spaces and flagellate chambers, and into the 

 central cavity, and leaves through the oscula. 



From Coelenterata we select Aurelia, Fig. 75, a Scyphomedusan 

 very commonly met with round our coast, swimming on the surface 

 of the sea. 



The mouth of Aurelia is surrounded by four perradial processes, 

 and the manubrium is short. The sense organs are modified 

 tentacles, which bear endodermal otocysts and ectodermal pigment 

 spots, or eyes. An aboral and an oral pit, both lined by special- 

 ised epithelium on the surface of the disc, are regarded as olfactory. 



In the order Hexactinia belonging to this division are the 

 Madreporaria. They are soHtary or colonial ; their most remark- 

 able characteristic is their power of secreting a calcareous skeleton. 

 The Madreporaria are of the greatest importance in the history 

 of the earth. They are the true corals, and their skeletons form 

 by far the greater part of the coral rock which has built up a 

 considerable portion of the earth's crust. Reef-forming corals do 

 not, as a rule, grow below the forty-fathom line, and are not 

 usually found north or south of a belt extending 30*^ each side of 

 the equator. 



Chapter VI. treats of Platyhelminthes, which are divided into 

 three classes : Turbellaria, Cestoda, and Trematoda. The Tur- 

 bellaria are subdivided into Rhabdocoelida and Dendrocoelida. 

 In the latter division are found the genera Tricladia and Poly- 

 cladia. We have selected Fig. 76, showing Plan of a Polyclad. 



In the Polyclads there are many, usually eight, nerve cords, 

 which diverge from Ihe central cerebral ganglion (Fig. 76). 

 Sensory cells, provided with tactile hairs, occur in the ectoderm. 

 The eyes are usually two or four in number, but may be more 

 numerous, and they increase by division. Auditory vesicles also 

 occur, they are often single, and consist of a vesicle full of fluid, 

 in which a calcareous otolith floats. The anterior end of the 

 body is remarkably sensitive, and in some genera forms a tactile 

 proboscis, which can be retracted into a sheath. 



To the third class of this division, the Trematoda, belong the 

 Liver Fluke, Fasciola ( Distoma) hepaiica^ (Figs. 77 and 78), and is 

 found in the liver of diseased sheep. It is about three-quarters of 

 an inch long, and has a flattened, leaf-like shape. 



