58 ZOOLOGY 
no mouth, they bear short knobbed branches, or tentacles, in 
the Milleporidae. 
The generative cells of Jillepora, which is hermaphrodite, 
arise in the ectoderm of the coenosarcal tubes, which connect 
the gastrozoids and the dactylozoids. The spermospores or cells 
which are destined to break up into spermatozoa, migrate 
through the mesogloea into the endoderm, and then travel along 
the tubular coenosare to a hydranth, usually a dactylozoid. 
They pass into the cavity of the hydranth, and then re-enter 
the endoderm, and push out the mesogloea into a diverticulum 
between the tentacles. The number of the diverticula or 
sporosacs on each dactylozoid varies from one to five. The 
spermatozoa mature in the sporosacs. The ova make their way 
in a similar manner through the mesogloea into the endoderm ; 
here they attach themselves by a stalk to the inside of the 
mesogloea and grow considerably. The ovum may wander 
about to seek a more favourable position for fertilisation, which 
probably takes place in the lumen of the canals. Similar 
wanderings of the sexual cells take place among the Hydro- 
medusae, 
Order 4. Siphonophora. 
CHARACTERISTICS.— These are HYDROMEDUSAE which live together 
in colonies floating at or near the surface of the sea (PELAGIC). 
The members of the colones are hydriform individuals alone 
(VELELLA),o7 hydriform and medusiform, they have undergone 
great modifications, so that in each colony a great number of 
persons are present performing various functions, and exhibit- 
ing a great diversity of form. 
The chief modifications are as follows: (.) the Gastrozoid, 
whose function is to absorb nutriment. It has a mouth, and 
communicates at its aboral end with the tubular coenosare 
connecting the various persons. (i1.) Hydrophyllia: these are 
protective covering-pieces, which have a central endodermic 
canal, and are usually borne on the coenosare. (1.) Tentacles : 
these may be short and tubular, or may attain the length of 
many feet (Physalia). They are usually branched, and each 
branch is provided with a battery of nematocysts. (iv.) Dac- 
tylozoids: hydroid structures without a mouth, and usually 
