. The Cambridge natural history. Zoology. 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 t
. The Cambridge natural history. Zoology. 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\ 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
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