COMPOSITE SEA-SQUIRTS 297 



one with a microscope to see right into the living 

 creature and to watch the beating of its heart, the 

 movement of its colourless, clear blood, and the lashing 

 of the minute hair-like whips set on the trellis-work of its 

 gullet by which the sea-water is drawn in at the mouth 

 and passed out through the perforations of the gullet wall 

 — bringing food and rich streams of oxygenated liquid to 

 the little animal. 



That is one kind of Ascidian not far removed from the 

 common rough sea-squirt. Now we will go a step further. 

 Suppose that instead of merely budding new individuals 

 from its stem-like base, the Ascidian is liable to a budding 

 process which affects its whole body. This is what 

 happens in some kinds of Ascidians : each original indivi- 

 dual, as it grows, becomes divided or " budded " into some 

 six or a dozen closely adherent individuals, each with its 

 own mouth, and otherwise complete. But all are united 

 to one central peri-branchial out-flow chamber by the orifice 

 of which the water, taken in by all, is passed to the 

 exterior, and all are enclosed in a continuous sac, coat or 

 tunic. Such composite Ascidians exist in great variety. 

 Some are upright, and an inch or more in length, and 

 often beautifully coloured — tinted pink or violet — others 

 are flattened, and the united individuals are spread star-like 

 around their central branchial orifice, on the rock to which 

 their common tunic adheres. Not only that, but a whole 

 series of such star-like composite groups is formed, scat" 

 tered at intervals (by separation after budding) in a sheet 

 of encrusting tough, gelatinous tunic. The gelatinous 

 encrusting " sheet " is the same thing as the tough sac of 

 the common simple Ascidian, This is the nature of the 

 beautiful encrusting growths known as " Botryllus," which 

 are found either on rocks or stones, or on large 

 seaweeds (Fig. 33). The flat, star-like composite 

 individuals are one third of an inch across, and are 



