58 ZOOLOGY 



no mouth, they bear short knobbed branches, or tentacles, in 

 the Milleporidae. 



The generative cells of Millepora, 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 coenosarc 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. 



Chaeacteeistics. — These are Hydkomedusae whicli live together 

 in colonies floating at or near the surface of the sea (PELAGic). 

 The members of the colonies are hydriform individuals alone 

 (VeIiELLa), or hydriform and medusiform; theyhave undergone 

 great modifications, so that in each colony a great number of 

 persons are present performing various fionctions, and exhibit- 

 ing a great diversity of form. 



The chief modifications are as follows : (i.) the Gastrozoid, 

 whose function is to absorb nutriment. It has a mouth, and 

 communicates at its aboral end with the tubular coenosarc 

 connecting the various persons, (ii.) Hydrophyllia : these are 

 protective covering -pieces, which have a central endodermic 

 canal, and are usually borne on the coenosarc. (iii.) 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 



