REPORT ON THE TUNICATA OF PLYMOUTH. 67 



the abundance and height of the foreign organisms competing with 

 them for space and oxygen, resembling in this respect numerous 

 epiphytes and other vegetable growths in a thick Brazilian forest. 



If the smaller zooids of a living colony be touched with a needle, 

 the bright yellow thorax frequently withdraws itself completely from 

 the greenish test of that region and disappears within the stalks or 

 below the level of the corm. The larger zooids contract upon irritation, 

 but do not completely withdraw in this way. On contraction they 

 give, as it were, a stoop or bend towards the dorsal side — away from 

 the side with the line of yellow pigment ; this is due to the fact that 

 the longitudinal muscle-bundles are somewhat more numerous in the 

 dorsal than in the ventral section of the body. Very rarely, if the 

 irritation be continued, the larger zooids may also behave like the 

 smaller ones. 



Since I made these experiments with Pycnoclavella I have found 

 that Forbes and Goodsir* noticed a precisely similar reaction in the 

 case of their Syntethys hebriclicus. Indeed, there are several interesting 

 resemblances between the genera Diazona {Syntethys) and Pycnoclavella, 

 the chief of which I may mention as being the greenish colour of the 

 test and the embedding of the abdominal regions of the zooids in a 

 thick basal mass of common test, the thoracic portions remaining 

 free ; in Diazona this process is more complete than in Pycnoclavella. 



With regard to the relations of this species to Clavelina, I have 

 stated above that although there can be no doubt that both Stereo- 

 clavella and Pycnoclavella are closely allied to that genus, and, indeed, 

 almost certainly derived from it, I believe others will agree with me 

 that this species is more closely related to Glavelina itself than to the 

 species of Stereoclavella ; its nearest ally seems to be Milne-Edwards' 

 species Glavelina producta.f This species produces buds from the 

 lateral walls of the abdomen as well as from the basal stolonial tubes, 

 a fact hitherto without parallel in the Clavelinidee. Pycnoclavella 

 atirilucens, however, exhibits occasionally the same phenomenon (see 

 PI. II, fig. 2), and there can be little doubt that the stolonial tubes 

 traversing the foot-stalks in this species, whether they remain sterile 

 or produce buds, are the direct homologues of the fertile stolonial 



* Trans. "Roy. Soc. Edin., xx, 1853, p. 308. 



t Milne-Edwards' Observations, 1. c, p. 267, pi. ii, fig, 3. 



