308 : THE OCEAN WORLD. 
all the species of Mustra, 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 algze 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 (/ fofiacea, 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 AMi/epora, but the structure of 
the animal is quite enough to distinguish them. £. 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 Zunicata 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, another 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 
