308 THE OCEAN WORLD. 



all the species of Flustra, of Eschara, and other now well-known 

 genera. 



All the species of the genus Flustra are marine, whose integument 

 in hardening forms a thin shell of a horny appearance ; their little 

 cells, more or less horny, are often grouped symmetrically, somewhat 

 like the cells in a bee-hive. Sometimes they form a crust which 

 covers algae and other marine bodies ; sometimes they form ribbon- 

 like stems. In some species the cells are only found on one side ; in 

 others they occupy both. Their orifices are extremely small, and are 

 often defended by spines quite microscopic (F. foliacea, Fig. 122). 



Their tentacles, like other Polyzoa, are covered with cilia, always 

 vibratile, disposed in a straight line, which in their movements pro- 

 duce the effect which a row of animated pearls might be supposed to 

 produce if rolled upwards from the base to the summit of the organ. 



Some species of the genus Eschara form quite shrub-like masses, 

 calcareous in structure, the polyp cell being imbedded in the mass. 

 Some of them may be very easily, by superficial observers, mistaken 

 for species of the Zoantharian genus Millepora, but the structure of 

 the animal is quite enough to distinguish them. E. cervicornis is 

 common around the coasts of Great Britain and Ireland. As it is 

 with the corals, so it is here ; each eats for the benefit of itself and 

 for the community labour and nutrition for the community, labour 

 and food for itself. 



MOLLUSCOIDA TUNICATA. 



On seeing one of the Tunicata for the first time, a stranger to 

 zoology would scarcely take them for animals at all. Almost always 

 attached to submarine rocks, these beings have the form of a simple 

 sac. Their skin, gelatinous, or horny, is at times covered over with 

 marine plants and polyps. They have neither arms, nor feet, nor 

 head; but then they have a mouth, placed at the entrance of a 

 digestive tube, and, in connection with the latter, a special opening 

 intended for the excreta. The mouth is at the bottom of a great cavity, 

 the walls of which are covered with blood-vessels ; for this cavity is 

 the seat of respiration, and is covered with vibratile cilia. Thus the 

 same canal serves first for respiration, and then, as an entrance to the 

 cavity for digestion, anotner instance of the economy of Nature. 

 Another remarkable fact in connection with their circulation is found : 

 their heart is the centre of a well -developed vascular system, but 

 unlike what is usually found in animals, the blood which traverses it 

 takes such a course that, in the space of a few minutes, the auricle 

 and ventricle of the heart become changed into ventricle and auricle 



