SPONGES AND ZOOPHYTES. 127 



from it. The ova, when fertilized by the spermatozoa, undergo 'seg- 

 mentation ' according to the ordinary type ( 581), the whole yolk-mass 

 subdividing successively into 2, 4, 8, 16, 32 or more parts, until a 

 'mulberry mass' is formed; this then begins to elongate itself, its surface 

 being at first smooth, and showing a transparent margin, but afterwards 

 becoming clothed with cilia, by whose agency these little planulce, closely 

 resembling ciliated Infusoria, first move about within the capsule, ancl 

 then swim forth freely when liberated by the opening of its mouth. At 

 this period the embryo can be made out to consist of an outer and an 

 inner layer of cells, with a hollow interior; after some little time the cilia 

 disappear, and one extremity becomes expanded into a kind of disk by 

 which it attaches itself to some fixed object; a mouth is formed, and 

 tentacles sprout forth around it; and the body increases in length and 

 thickness, so as gradually to acquire the likeness of one of the parent 

 polypes, after which the ' polypary ' characteristic of the genus is gradu- 

 ally evolved by the successive development of polype-buds from the first- 

 formed polype and its subsequent offsets. The Medusae of these polypes 

 (Fig. 358) belong to the division called f naked-eye/ on account of the 

 (supposed) eye-spots usually seen surrouniing the margin of the bell at 

 the base of the tentacles. 



518. A characteristic example of this production of Medusa-like 

 'gonozooids' is presented by the form termed Syncoryne Sarsii (Fig. 

 356) belonging to the sub-order Athecata. At A is shown the alimentary 

 zooid, or polypite, with its tentacles, and at B the successive stages a, b, 

 c, of the sexual zooids, or medusa-buds. When sufficiently developed, 

 the medusa swims away, and as it grows to maturity enlarges its manu- 

 brium, so that i t hangs below the bell. The Medusse of the gen us Syncoryne 

 (as now restricted) have the form named Sarsia in honor of the Swedish 

 naturalist Sars. Their normal character is that of free swimmers; but 

 Agassiz ascertained that in some cases, towards the end 'of the breeding 

 season, the sexual zooids remain fixed, and mature their products while 

 attached to the zoophyte. 1 This condition of the sexual zooids is very 

 common amongst the Hydroida; and various intermediate stages may be 

 traced in different genera, between the mode in which the gonozooids are 

 produced in the common Hydra, as already described, and that of Syn- 

 coryne. In Tulularia the gonozooids, though permanently attached, 

 are furnished with swimming bells, having four tubercles representing 

 marginal tentacles. A common and interesting species Tiibularia, indi- 

 visa receives its specific name from the infrequency with which branches 

 are given off from the stem, these for the most part standing erect and 

 parallel, like the stalks of corn, upon the base to which they are attached. 

 This beautiful Zoophyte, which sometimes grows between the tide-marks, 

 but is more abundantly obtained by dredging in deep water, often attains 

 a size which renders it scarcely a microscopic object; its stems being 

 sometimes no less than a foot in height and a line in diameter. Several 

 curious phenomena, however, are brought into view by Microscopic 

 examination. The Polype-stomach is connected with the cavity of the 

 stem by a circular opening, wjiich is surrounded by a sphincter; and an 

 alternate movement of dilatation and contraction takes place in it, fluid 

 being apparently forced up from below, and then expelled again, after 

 which the sphincter closes in preparation for a recurrence of the opera- 

 tion; this, as observed by Mr. Lister, being repeated at intervals of 



1 Hincks, op. cit., p. 49. 



