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 



