SECT, iv PHYLUM CCELENTERATA 119 



growth on the wooden piles of piers and wharfs. It consists of 

 branched filaments about the thickness of fine sewing-cotton : of 



\these, some are closely adherent to the timber, and serve for 

 aJrtachment, while others are given off at right angles, and present 

 a^lntervals short lateral branches, each terminating' in a bud-like 

 enlargement. 



^e structure is better seen under a low power of the 

 microscope. The organism (Fig. 83) is a colony, consisting of a 

 common stem or axis, on which are borne numerous zooids. The 

 - axis consists of a horizontal portion, resembling a root or creeping 

 stem, and of vertical axes, which give off short lateral branches in 

 an alternate manner, bearing the zooids at their ends. At the 

 proximal ends of the vertical axes the branching often becomes 

 more complex : the offshoots of the main stem, instead of ending 

 at once in a zooid, send off branches of the third order on which 

 the zooids are borne. In many cases, also, branches are found to 

 end in simple club-like dilatations (Bd. 1, %)\ these are immature 

 zooids. 



The large majority of the zooids have the form of little conical 

 structures (P. 1 P. 4), each enclosed in a glassy, cup-like invest- 

 ment or hydrotlieca (h.th), and produced distally into about two 

 dozen arms or tentacles (t) : these zooids are the polypes or Jiydrantlis. 

 Less numerous, and found chiefly towards the proximal region of 

 he colony, are long cylindrical bodies or blastostylcs (bis), each 

 ^closed in a transparent case, the gonotheca (g.tJi), and bearing 

 numerous small lateral offshoots, varying greatly in form according 

 to their stage of development, and known as medusq-luds (m.bd). By 

 studying the development of these structures, and by a comparison 

 with other forms, it is known that both blastostyles and medusa- 

 buds are zooids, so that the colony is trimorphic, having zooids of 

 three kinds. 



To make out the structure in greater detail, living specimens 

 should be observed under a high power. A polype is then seen 

 to consist of a somewhat cylindrical, hollow body, of a yellowish 

 colour, joined to the common stem by its proximal end, and pro- 

 duced at its distal end into a conical elevation, the manubrium or 

 hypostome (mnb), around the base of which are arranged the twenty- 

 four tentacles in a circle. Both body and manubrium are hollow, 

 containing a spacious cavity, the enteron (cnt), which communicates 

 with the outer world by the mouth (mth\ an aperture placed at 

 the summit of tho manubrium. The mouth is capable of great 

 dilatation and contraction, and accordingly the manubrium appears 

 LOW conical, now trumpet-shaped. Under favourable circum- 

 tances small organisms may be seen to be caught by the polypes 

 and carried towar Is the mouth to be swallowed. 



i h< , \ i ne-glass, and 



is perfectly trai and coK> A short distance froi<; its 



