66 INVERTEBRATE ANIMALS. 



embryos are not developed into Medusae, such as produced 

 the ova and sperm-cells, but straightway grow up into the 

 plant-like, sexless colony, from which the medusiform gono- 

 phores were originally budded forth. In these cases, there- 

 fore, the individual Hydroid consists of a fixed, rooted colony 

 or trophosome, producing fresh zooids by a process of budding, 

 but incapable of producing the essential elements of reproduc- 

 tion, together with a free and independent series of generative 

 buds, or gonosome, in which the elements of reproduction are 

 developed. 



OEDER III. SEKTULAEIDA. In this order of the Hydroida 

 we have the most familiar and best known of all our zoophytes 

 namely, the sea-firs and their allies. The horny, plant-like 

 polyparies of the Sertularida are familiar to every visitor at 

 the sea-side, and by those unacquainted with their true 

 nature they are almost universally set down as sea-weeds. 

 The Sertularida are very closely allied to the compound forms 

 of the Cwynida, resembling them in being rooted, plant- 

 like colonies, composed of a number of similar polypites or 

 zooids, produced by budding from a primitive zooid. As in 

 the Tubularians among the Corynida, the whole ccenosarc is 

 enveloped in a horny or chitinous envelope or polypary 

 (Fig. 17, a), and this is the structure which is most familiarly 

 known to sea-side observers. The Sertularida, however, are 

 distinguished from the Corynida by two points: Firstly, 

 none of the Sertularida are simple, but are all compound, con- 

 sisting of more or less numerous polypites, united by a 

 branched ccenosarc. Secondly, the polypary of the Sertularida 

 differs from that of the Corynida in not simply reaching to 

 the bases of the polypites, but in being prolonged to form a 

 number of little cups or " hydrothecae " (Fig. 17, a, b) within 

 which the polypites are lodged. Each polypite has a cup of 

 its own, within which it can entirely withdraw, and from 

 which it can protrude its free extremity. 



The polypites of the Sertularida have essentially the same 

 structure as in the Corynida, and each may be compared to a 

 little Hydra. Each, namely, consists of a soft, contractile, and 

 extensile body, which is furnished at its free extremity with 

 a mouth and a circlet of prehensile tentacles, richly furnished 

 with thread-cells. The mouth opens into a chamber which 

 occupies the whole length of the polypite, and which is to be 

 regarded as the combined body-cavity and digestive sac. At 

 its lower end this chamber opens by a constricted aperture 



