EEPORT ON THE TUNICATA OF PLYMOUTH. 55 



interesting fact that the stalks of the zooids are elongated in direct 

 proportion to 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 irri- 

 tation, 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 nume- 

 rous 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 Jiehridicus. 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 po-oducta.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 Clavelinidse. 

 Pycnoclavella aurilucens, 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 tubes of the abdominal walls in Clavelina 

 producta. The only other '' social Ascidian " possessing morpho- 

 logically similar structures is Sluiteria rubricollis (Van Beneden| 



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



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



X E. van Beneden, Lef: genres Ecteinascldia, RhopaJiea et Sluiteria ; note pour servir a 



