PENNATULIDA. 



youngest polyps in each transverse series are those nearest to this 

 line; the median line and its more immediate surroundings is often 

 also sterile> of polyps; when this is the case the naked streak, how- 

 ever, is narrower here than on the opposite surface, with the exception 

 of the lower part towards the peduncle (Renilla, however, is a remark- 

 able exception). Thus the two median surfaces of the stem are gener- 

 ally easily distinguished '). Making the median line our starting-point 

 we might say that the surface D shows a centrifugal* development 

 of the polyps, the surface V a centripetal* one; so far we might well 

 abandon the terms ventral and dorsal side, and instead of them use 

 centrifugal and centripetal side; these latter names, however, are 

 somewhat clumsy in descriptions, as is also the case, I think, with 

 the terms proposed by Bourne 2 ): prorhachis (for D) and metarhachis 

 (for F"); dorsal and ventral side are, and will always be, the most 

 handy names, if only a rational choice were agreed upon. In my 

 choice I have started from the following consideration: the stem of 

 the sea-pens, rhachis -j- peduncle, is the direct prodtict of the individual 

 developed from the egg (the oozooite of Lacaze Duthiers); this 

 primary individual is for some time a solitary polyp of the typical 

 octocorallian structure ; accordingly, it is bilaterally symmetrical with 

 respect to a plane naturally called a dorso-ventral plane; the mouth 

 and pharynx (stomodaeum) are oval with the longer axis in the plane 

 of symmetry; this plane bisects two median chambers, mutually diffe- 

 rent and differing from all the others: only one of them contains re- 

 tractor muscles, and this one is on the same side as the ciliated groove 

 (sulcus, siphonoglyphe3)) of the pharynx; it has long ago been agreed 

 on calling this sulcar side the ventral side in all polyps of octo- 

 corals. If we will keep this designation here, we must accordingly 

 be consistent, and also use it for the corresponding, homologous side 

 of the stem of the sea-pens. Several years ago by following the 

 development of Pennatula phosphorea, I have shown that the sulcar 

 side- of the primary polyp is really the side F, the centripetal side; 

 only this side, then, can justly be called the ventral side. 



Whichever appellation, however, is preferred it is obvious that 

 in all Pennatulids the same name is to be used for the same (homo- 

 logous) side. This, K 6 Hiker (and with him the later authors) has 



Fig. 2. 



Fig- 3' 



Young specimens of Renilla reni- 

 formis (Pall.) and Pennatula phos- 

 phorea L. , seen from the dorsal- 

 side; about 2',2 times the natural 

 size. P terminal or primary polyp, 

 /, p' lateral polyps, p' younger than 

 p\ ^terminal zooid, z dorsal zooids ; 

 z' (only in Renilla] zooids placed on 

 the polyps; n naked dorsal streak. 

 The arrows indicate that the polyps 

 grow in the direction of the unseen 

 ventral side of the colon}-. 



') Even in such a form as Dixtichoptilum (PI. I, Figs. 12 14) or in young stages for instance of Virgularia (PL II, 

 Fig- 2 5)- The polyps placed in a single series on either side of the rhachis will always incline somewhat towards the side V. 



2 ) Anthosoa in Ray Lankester: Treatise on Zoology. Part II. 1900. p. 31. 



3) The ciliated groove, as is well known, is not developed in the older polyps of the sea-pens, but very highly 

 developed in the zooids and the quite young polyps (Hickson, Phil. Tr. 1883). 



