The peculiarities of the order are due to the great growth and modification in structure of the first formed zooid of the colony. This zooid (Oozooid, Hauptpolyp, or Axial zooid) increases greatly in length, develops very thick fleshy walls, usually loses its tentacles, digestive organs, and frequently its mouth, exhibits profound modification of its system of mesenteries, and in other ways becomes adapted to its function of supporting the whole colony. [Illustration: FIG. 158.—Diagram of a Sea-pen. _L_, leaves composed of a row of autozooids; _R_, rachis; _St_, stalk; _T_, anthocodia of the axial zooid, usually suppressed. (After Jungersen.)] The axial zooid shows from an early stage of development a division into two regions: a distal region which produces by gemmation on the body-wall numerous secondary zooids, and becomes the rachis of the colony; and a proximal region which becomes the stalk or peduncle, and does not produce buds (Fig. 158). The secondary zooids are of two kinds: {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 increases 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[385] (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 (_Umbellula_), scattered on the dorsal side of the rachis (_Renilla_, Fig. 160), or 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 (_Pteroeides_), but more frequently between the leaves or rows of autozooids, or scattered irregularly among the autozooids. Usually the siphonozooids are of one kind only, but in _Pennatula murrayi_ there is one specially modified siphonozooid at the base of each leaf,[386] which appears to have some special but unknown function. [Illustration: FIG. 159.—Diagram of a portion of a rachis of a Sea-pen, _aut_, The rows of autozooids; 1-6, the order of age of the autozooids composing a leaf; _D_, the dorsal side of the rachis; _Si_, the siphonozooids; _V_, the ventral side of the rachis. (After Jungersen.)]