156 THE BOOK OF NATURE STUDY 



(c) THE SEA-SQUIRTS OR TUNICATES 



The strange animals known as Tunicates or Ascidians, more 

 popularly as sea-squirts, are well represented around our coasts. 

 They are sometimes found attached to large sea-weeds or to the 

 rocks near the low-tide mark, and on some coasts they are occa- 

 sionally thrown up by storms. A typical form can be recognised, 

 at once by the glassy tunic or " test " which covers the irregul- 

 arly oval body. One end of the body is attached to stones or 

 weed ; the other is more tapering, and bears the mouth at the 

 apex, and an exhalant aperture to one side. There is a suggestion 

 of a double-mouthed leather bottle. During life water is con- 

 tinually being drawn in by the mouth bearing microscopic 

 food particles and oxygen with it and passed out at the ex- 

 halant aperture. If irritated the animal often expels a jet of 

 water with considerable force from the mouth or from both 

 apertures, whence the name sea-squirt. The gelatinous test 

 which covers the body is of much interest, inasmuch as it con- 

 sists in great part of a substance practically identical with cellulose, 

 the characteristic building material of plants. Thus in the 

 most sluggish part of a sluggish animal we find a characteristic 

 vegetable substance. 



The Ascidians are difficult to understand ; they are, so to 

 speak, Vertebrates in disguise. Except in a few free-swimming 

 forms their real nature is disguised by a degeneracy which befalls 

 them in early youth. The larval Ascidian lives in the open sea, 

 like a miniature model of a very young tadpole, swimming about 

 by means of a delicate tail. It has most of the essential verte- 

 brate characters, a dorsal nerve-cord with a brain, a supporting 

 rod or notochord in the tail region, gill-slits opening from the 

 pharynx to the exterior, and an eye developing from the brain. 

 But in most cases the promise of youth is unfulfilled ; the larva 

 after a short period of free swimming fixes itself by its head to 

 stone or weed, settles down to sedentary life, loses tail and tail- 

 rod, nerve cord and eye, and becomes strangely deformed. They 

 are much too difficult for ordinary school study, but if they are 

 found on the beach or attached to the rocks the general fact 

 should be made clear that they are animals which stumble on the 



