ALCYONARIA — PENNATULACEA 



359 



the autozooids and the siphonozooids. The former have the 

 ordinary characters of an Alcyonarian zooid, and produce sexual 

 cells ; the latter have no tentacles, a reduced mesenteric system, 

 and a stomodaeum provided with a very wide siphonoglyph. 



The arrangement of the autozooids and siphonozooids upon 

 the axial zooid is subject to great modifications, and affords the 

 principal character for the classification of the order. In the 

 Pennatuleae the autozooids are arranged in two bilaterally 

 disposed rows on the rachis, forming 

 the leaves or pinnae of the colony (Fig. 

 158). The number in each leaf in- 

 creases during the growth of the colony 

 by the addition of new zooids in regular 

 succession from the dorsal to the ventral 

 side of the rachis^ (Fig- 159). In 

 other Pennatulacea the autozooids are 

 arranged in rows which do not unite to 

 form leaves (Funiculina), in a tuft at 

 the extremity of a long peduncle 

 ( Umhellula), scattered on the dorsal side 



y\jsj. 



i-ai/t 



of a rachis of a Sea-pen. aut, 

 The rows of autozooids ; 1-6, 

 the order of age of the axito- 

 zoolds composing a leaf ; D, 

 the dorsal side of the rachis ; 

 Si, the siphonozooids ; V, the 

 ventral side of the rachis. 

 (After Jungersen. ) 



of the rachis {Renilla, Fig. 160), or Fig. 159.— Diagram of a portion 



scattered on all sides of the rachis 



(Cavernularia, Fig. 161). In those 



forms in which the autozooids are 



scattered the bilateral symmetry of the 



colony as a whole becomes obscured. 



The siphonozooids may be found on the 



leaves (Fteroeides), but more fretpently between the leaves or 



rows of autozooids, or scattered irregularly among the autozooids. 



Usually the siphonozooids are of one kind only, but in Fen- 



natula murrayi there is one specially modified siphonozooid at 



the base of each leaf,^ which appears to have some special but 



unknown function. 



In Umhellula gracilis each siphonozooid bears a single pinnate 

 tentacle, and in some other species of the same genus there is 

 a tentacle which is not pinnate.^ 



1 Jungersen {Danish Ingolf Expedition, Pennatulida, 1904) has shown that this 

 is the correct nomenclature of the regions of the rachis. Nearly all other authors 

 describe the dorsal side as ventral and the ventral as dorsal. 



2 S. J. Hickson, Report British Association (Southport Meeting), 1903, p. 688. 

 ' Marshall, Trans. Roy. Soc. Edinb. xxxii. 1883, p. 143. 



