THE TUNICATED MOLLUSCS. 



14-5 



remain in the vicinity, playing round and round, until the or- 

 ganic matter is quite consumed. Now, a tiny annelide or other 

 animal, caught by the " bird's-head " of the polyzoa, and tightly 

 held, would presently die ; and though, in its own substance, it 

 would not yield any nutriment to the capturer, yet by becoming 

 the centre of a crowd of busy infusoria, multitudes of which 

 would constantly be drawn into the tentacular vortex and swal- 

 lowed, it would be ancillary to its support, and the organ in 

 question would thus play no unimportant part in the economy 

 of the animal.' 



Besides the 'avicularise,' most of the polyzoa are furnished 

 with 'vibracula,' long bristle-shaped organs, each one springing 

 at its base out of a sort of cup that contains muscles by which 

 it is kept in almost constant motion, sweeping slowly and care- 

 fully over the surface of the polyzoary, and removing what 

 might be injurious to the delicate inhabitants of the cells when 

 their tentacles are protruded. So carefully have all the wants 

 of these humble creatures been provided for ! 



The nearest relations of the polyzoa 

 are the tunicated mollusca, so called 

 because their soft parts are enclosed, 

 not in a shell, like the majority of 

 the class, but in a tough leathery 

 coat or tunic. To this subdivision be- 

 long the various kinds of Ascidise, both 

 simple and compound, the Salpaa and 

 the Pyrosomas. The simple Ascidia?, 

 or sea-squirts, are sometimes found at- 

 tached to sea-weeds and stones in the 

 littoral zone, while others are fre- 

 quently thrown up from deeper water 

 on the beach. Some are of a large size, 

 several inches in length. Their outer 

 form is that of a bag with a smooth or 

 variously roughened semi-transparent 

 skin, furnished with two small open- 

 ings, from which, on the slightest pres- 

 sure, a jet of water is sent to a con- 

 siderable distance. These creatures lead a very inactive life. 

 Adhering by their base to rocks, shells, and other submarine 



L 



Ascidia Mammillata . 



a branchial orifice, open, b anal 



orifice, closed. 



