16 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



into the organism, either through weakening occasioned by too frequent 

 repetition of the isosporulation, or else from external life-conditions 

 of an unfavourable nature. 



It is worthy of note that the tendency among colonial forms 

 is towards the suppression of a skeleton. Collozoum has none ; 

 Sphcerozoum has loose spicules only. The reason probably is the 

 hindrance to the formation of new individuals by the fission of the 

 central capsule, which a hard resistent casing to the latter would 

 entail. With loose spicules, if the central capsule divides, then each 

 half simply takes its share of the spicules with it. 



Distribution. All latitudes know the Radiolarians, but they 

 abound most in the warm seas between the tropics. The majority 

 are pelagic ; Sjihcerozowm and GollozoiiTii among the number. Many 

 of the great depths of the sea are covered by ooze formed all but 

 entirely of their decaying remains, e.g. 2 — 3000 fathom depths of the 

 Pacific and Indian Oceans. To other deep ocean oozes, the Red-clay 

 deposits, and the Globigerina-ooze, they contribute largely. The 

 familiar Tripoli powder, used for polishing, consists largely of their 

 remains ; many fine whetstones are slaty rock formed in great part of 

 their siliceous skeletons. In many lands they largely compose 

 certain clays and marls, thus indicating, albeit in fragmentary 

 manner, the localities of uprisings of some ancient sea-bottoms of great 

 depths. Deposits in the Barbadoes and the Nicobar Islands, in 

 Algeria and Greece, are the best known of these— and it is from such 

 localities, especially from the first-named, that are obtained by careful 

 washing and separation, those beautiful fossil forms so well known as 

 microscopical preparations. These are all of Cainozoic age, but other 

 fossil species date from early Palaeozoic times, while in Jurassic rocks, 

 they even form quartzite, so compactly are they knit together. 



Study XV. — The Hydroid stage of Obelia geniculata. 



Obelia constitutes a very typical form of Thecate Zoophyte, and 

 a study of the slide now sent out, taken in conjunction with reference 

 to Study XI, and Plate IX in last volume, will furnish materials for 

 a ready comprehension of the essential details of the anatomy and of 

 the life-history. 



Study XVI. — The stalked larva of Antedon. 



Few of us are unfamiliar, at least by name, with Antedon, the 

 Rosy Feather Star. The extreme elegance of its long slender arms 

 has long made it famous among the artistic triumphs of the world of 



