CHORDATA 243 



student will do well to collect and preserve specimens for 

 this purpose. 



The Zostera fields are another good hunting ground for 

 the compound ascidians, many forms are abundant there 

 which are scarce elsewhere. Besides the Botrylloides 

 already mentioned he will find large, almost flocculent, 

 masses of a light grey colour. These are colonies of 

 Aplidium gelatinosum, pealike clusters of Aplidium 

 zostericola, and many others, some difficult to distinguish 

 from the egg clusters of some of the gastropod molluscs 

 which occur amongst them. 



In these compound ascidians the reproduction, from the 

 original founder of a colony, is chiefly by budding (gemmi - 

 partite reproduction), but, especially in autumn, they all 

 give rise to a " tadpole " generation. These little " tad 

 poles," swimming away, and finally settling, form new 

 colonies. 



(The nomenclature given above is after Giard.) 



Tn the simple or solitary ascidians the field is not so 

 large. 



Tn rock crevices, and at the foot of rocks in oozy, low- 

 tide situations, may be found, very conspicuously exposed, 

 the large dona intestinalis (Fig. 103). It is of a pale green 

 colour, with the protrusible part, the tubes, of the apertures 

 still paler, nearly white. 



The test of this ascidian is very thin and membraneous, 

 the whole animal almost gelatinous to the feel. It is 

 usually about three inches long, but in favourable situations 

 may grow to a length of six or eight inches. 



Of about the same size, but invested in a tough carti- 

 laginous tunic, with the apertures barely discernible, is 

 Ascidia mentula. This one is usually attached by the 

 entire side to rocks and stones, chiefly the under side of 

 the latter, at low-tide limit. 



A knife is required to collect these satisfactorily, as in 



