

 



130 ZOOLOGY 



SECT. IV 



growth on seaweeds, the wooden piles of piers, etc. It consists of 

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

 these, some are closely adherent to the weed or timber, and 

 serve for attachment, while others are given off at right angles, and 

 present at intervals short lateral branches, each terminating in a 

 bud-like enlargement. 



The structure is better seen under a low power of the 

 microscope. The organism. (Fig. 96) is a colony, consisting of a 

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

 consists of a horizontal portion (hydrorhiza) resembling a root or 

 creeping stern, 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, 2} : these are immature 

 zooids. 



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

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

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

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

 Less numerous, and found chiefly towards the proximal region of 

 the colony, are long cylindrical bodies or Uastostyles (bis), each 

 enclosed in a transparent case, the gonotheca (g.th), and bearing 

 numerous small lateral offshoots, varying greatly in form according 

 to their stage of development, and known as medusa-buds (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 inanubriurn are hollow, 

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

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

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

 dilatation and contraction, and accordingly the manubrium appears 

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

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

 and carried towards the mouth to be swallowed. 



The hydrotheca (h.th) has the form of a vase or wine-glass, and 

 is perfectly transparent and colourless. A short distance from its 



