COMPOUND ASCIDIA. 155 



When diffused superficially, the systems are more level than when 

 consolidated in a mass. 



But there is no uniformity either in the number of the individual 

 animals composing a system, nor in the figure of the different systems com- 

 posing an entire specimen of the Botryllus, as will be seen from the de- 

 tail. 



We are continually reminded of the unaccountable irregularities pre- 

 dominant among the productions of Nature, even when pertaining to the 

 same species, animal or vegetable. 



§ 1. Three old oyster shells, invested by the BotryUw verrucosus, 

 were found near the eastern side of Cramond, an islet in the estuary of the 

 Forth. One of them, about three inches in diameter, was completely 

 covered by this product, in numerous and varied systems. — PI. XXXVIII. 

 fig. 1 . Besides those of definite formation, there were other gelatinous 

 portions, containing a multitude of yellow granules, dispersed throughout 

 their substance. — Fig. 2. When viewed in profile, the systems appeared 

 rising slightly above the level of the surface of the shell. — Fig. 3, enlarged. 



Here the systems might have been inscribed in circles, ellipses, or 

 surrounded by less regular outlines. They were composed of a row of 

 from four to ten or twelve ascidise, environing a common central orifice ; 

 or they formed a superficial mass of several indistinct and more irregular 

 rows. 



The circular systems approaching symmetry, or presenting a stellate 

 appearance, were about four lines in diameter, as in figs. 6, 7. The length 

 of the longest oval system was about six. 



The character of the different systems, and the number of ascidia? 

 composing them, will be sufficiently explicit on inspecting the various 

 figures which are selected from all three specimens. 



Each of the individual ascidia 1 is about the sixteenth of an inch in 

 length ; the body swelling above is penetrated by two orifices, a larger 

 and a smaller. The larger above, seems to be formed by four concave cir- 

 cular arcs ; the smaller, nearer the centre, is not conspicuous in all. Both 

 are horizontal. 



