360 Copepoda living as Messmates with Ascidians. [Sess. 



thoracic segment that the name Notodelphys was given by 

 Professor Allman to what is made the representative genus of 

 Sars's division Notodelphyoida. 



Having seen how closely these Copepoda are associated 

 with the life and fortunes of the Tunicata, the question 

 may be asked, What is likely to happen to the Copepoda 

 should the Ascidian die ? Dr Canu, speaking of some of 

 the more active species, says that, on the death of their 

 host, they are able to quit their shelter and proceed in quest 

 of another Ascidian, and while doing so, move freely about 

 on the bottom of the sea. But as they are seldom captured 

 in this free-living condition, it can only be on rare occasions 

 that they require to adopt this form of life. 1 Yet though 

 the more active and lively species may be able to look after 

 themselves in the manner described by Dr Canu, it is prob- 

 able that not a few will share the fate of their host. 



The alteration from a free-living to a semi-parasitic life has 

 doubtless taken place long ago, seeing that the change of 

 habitat has resulted in such a retrograde development as to 

 cause the structure both of the body and its appendages to 

 become so modified as to differ profoundly from that of the 

 free-living species to which their ancestors appear to have 

 been related. It is also interesting to find that in the early 

 larval stages of even the most degraded forms they still retain 

 some traces of their former free-living habits ; and, moreover, 

 it is during this early stage that such degraded forms usually 

 seek for, and select, a suitable host, which, when selected, has 

 henceforward to shelter and provide food for these unbidden 

 guests. 



The number of Copepod species that have been recorded 

 as messmates of the Tunicata is considerable, — so much so, 

 that I can claim only a limited acquaintance with them. The 

 species of Tunicata which have been described as the hosts of 

 these Copepoda are also fairly numerous, and include such 

 large forms as Ascidia mentula — already referred to, — Ciona 

 intcstinalis, specimens of which have been obtained " measur- 

 ing as much as a foot in length," 2 and also the small Cynthias, 

 Sty das, Botryllus, &c. 



1 ' Le8 Cope"podes du Boulonnais,' by Dr Eugene Canu, p. 187 (1892). 



2 « Jour. Plymouth Marine Biol. Assoc.,' vol. vii. (N.S.), No. 2, p. 296 (Dec. 1904). 



