58 W. A. HERDMAN. 



connus as being Compound Ascidians because tliey belong to the 

 family Polystyelidse in regard to wliicli it must be considered still 

 doiibtful whether the masses of Ascidiozoids are true colonies. But 

 although they may be colonial forms now, there can be no doubt 

 tliat phylogenetically the Polystyelidae are closely related to Ihe 

 subfamily Styelinae of the Cynthiidae, the subfamily to which both 

 Bathyoncus and Stijelopsis belong. So we arrive at the interesting 

 conclusion that Iho four gênera in wliich up to now atrial tentacles 

 hâve been noticed, although differing widely from on another in 

 appearance, structure and habitat, are yet phylogenetically rather 

 closely related. I think it not unlikely that atrial tentacles will be 

 found, if looked for, in other members of the groups Styelinse and 

 Polystyelidae. 



Another point : It is an interesting fact, and may hâve some 

 significance, that putting aside Bathyoncus mirahilis, in regard to 

 the conditions of life of which we know nothing — ail the five 

 species in which atrial tentacles hâve as yet been demonstrated 

 form either colonies or aggregations, z, e. they hâve numbers of 

 sraall individuals or ascidiozooids massed together. It is quite con- 

 ceivable that under thèse crowded conditions it may be advantage to 

 the animais to hâve the power (to return to tlie suggestion I made in 

 the « Cliallenger » Report) of frequently reversing the current of 

 water, or of using the atrial for a tirae as the inlialent aperture — 

 possibly for example because of being so placed amongst neighbours 

 that the atrial siphon is able to draw upon a purer supply of water, 

 — and in any such case the advantage of having the entrance to 

 the peribranchial cavity provided whit a circlet of tentacles is 

 obvions. 



Possibly JuLiN, in his récent investigations on Styelopsis grossu- 

 laHa, has already solved the problem of the meaning and use of 

 the atrial tentacles, and may he able either to support or demolish 

 the above suggestion. In any case I shall be much interested to 

 learn his views on the matter. 



Liverpool, 1 August 1892. 



