rOEM OF EODY OF CCELEI^TTEEATA. 97 



tlie stem, where they liave the form of stomachal tubes (stomachs, 

 sucking tubes) {B G n.) In some cases some of them are not fully 

 developed^ but form tubes closed at the end, which function as 

 " tentacles." 



3) Protective Persons (Hydrophyllia) : in these one can often 

 make out the Medusa-type quite clearly, but in other cases it is 

 much less distinct, and they then have the appearance of hyaline, 

 lamellar pieces, overlapping the persons described under 2, 4, and 5. 



4) Tentacular Persons: these form simple or elongated 

 filaments (grappling-lines), which are arranged in tufts, are capable 

 of great extension, and are provided with special urticating organs 

 (Urticating batteries). The primitive Medusa form can be made out 

 in a few only of these organs, and that faintly. 



5) Generative Persons : as in the Hydroid-Polyps, these may 

 be seen in various stages of development. Although it is in a very 

 few cases only that they are metamorphosed into Medusae that 

 become free (Velella — Chrysosmitra), the medusiform type is 

 very commonly well mai-ked among them. They are generally found 

 in racemose bunches, just as in the Tubularige. 



The arrangement of these very variously differentiated pei'sons of 

 the stock of the Siphonophora differs in the different divisions, while 

 the locomotor and the protecting persons are completely wanting 

 in many genera. In general the arrangement or distribution of the 

 polymorphous persons of the stock is observed to be very constant 

 in genera and species ; gemmation from the stock takes place on 

 one side only, the arrangement of the buds all round the stock being 

 due to its spiral twisting. This is the cause of the arrangement of 

 the nectocalyces in two or more rows, as well as of the grouping of 

 the other organs. Nutritive, generative, and tentacular individuals 

 are generally placed together in groups, in such a way that there is 

 one bract to a group. While in most Physophoridae, these groups 

 are very close to one another, they are set at greater distances 

 from one another in the Diphyidse (Fig. 33, A B), and each group is 

 composed of a certain number of persons which, by breaking off 

 from the stock, may become distinct individuals (Eudoxise). 



The anterior end of the stem, which is distinguished by the 

 presence of locomotor persons, becomes in many divisions par- 

 ticularly perfect, owing to the development of an air sac. This has 

 the functions of a hydrostatic organ, and causes the anterior end of 

 the body to be always directed upwards while the stock is at rest 

 (Physophoridae). (c a.) It has an opening to the exterior, which 

 can be closed, and by which air has been observed to escape. The 

 greater development of these bladders, which in most Physophoridae 

 are rather small, appears to cause a degeneration of the locomotor 

 buds of the stock; and thus there seems to be a kind of compensat- 

 ing arrangement, through which, however, the power of the colony 

 to move actively is diminished. Instead of swimming, it now is 

 driven through the water. The locomotor persons are, for example, 

 absent in Ehizophysa, in which the air sac is increased in size. By 



H 



