XXXU THE VOYAGE OF H.M.S. CHALLENGER. 



outer capsule-membrane is elevated in the form of a short cylindrical tube or " apertural 

 ring" (collare paraboscidis), the external margin of which bends inwards, and at the base 

 of the ring passes over into the delicate internal capsule membrane. Upon this apertural 

 ring is situated a longer or shorter "apertural cone" (paraboscis), which is a tubular, 

 cylindrical or conical, prolongation of the membrane, open externally. 



The peculiar capsule-openings of the PILEODARIA were first discovered and carefully described 

 by Hertwig in 1879 (L. N. 33, pp. 95, 107). He found in all the six genera which he examined 

 three openings, a main-opening at the basal pole of the main axis and two accessory openings, one 

 on either side of the apical pole ; hence he named the whole group " TRIPYLEA." This name, how- 

 ever, is not applicable to the numerous PBLEODARIA mentioned above, which have only a main 

 opening without any accessory openings, nor to those genera in which the number of the latter is 

 variable. I have, therefore, replaced Hertwig's designation by the term " CANNOPYLEA," which has 

 reference to the peculiar tubular form of the opening. This I find much more developed in many 

 PJLEODARIA than Hertwig has represented, and I must also, in certain particulars, dissent from his 

 delineation of the minute structure, although this is in the main remarkably accurate. 



61. The Nucleus. The nucleus, enclosed in the central capsule of all Radiolaria, 

 behaves in every respect like a true cell-nucleus, and thus lies at the base of the now 

 universal opinion, that the whole Radiolarian organism, in spite of its varied development 

 and remarkable variations, is unicellular and remains throughout life a true individual, 

 cell. This important theory is not invalidated by the fact that the nucleus undergoes 

 peculiar modifications in many groups, and in certain groups presents appearances seldom 

 or never seen elsewhere. 



62. Uninuclear and Multinuclear Radioloria (Monocaryotic and Polycaryotic). 

 All Radiolaria present two different conditions in respect of the behaviour of the nucleus, 

 since in their young stages they are uninuclear (monocaryotic), and in later stages multi- 

 nuclear (polycaryotic). This is readily explained by the fact that each individual Radio- 

 larian is developed from a simple unicellular swarm-spore, and that afterwards, before 

 the formation of swarm-spores, the single nucleus divides into many small nuclei. Thus 

 in the Radiolaria the nucleus is pre-eminently the organ of reproduction and inheritance. 

 The division of the originally single nucleus into many small nuclei may take place, how- 

 ever, at very different periods, so that the Radiolaria may be divided in this respect 

 into precocious and serotinous. 



63. Serotinous and Precocious Radiolaria. In the great majority of the Radiolaria 

 the division of the nucleus takes place only at a late period, a short time or even immediately 

 before the process of spore formation ; it then breaks up rapidly into numerous small 

 nuclei (always more than one hundred, sometimes many thousands), and each of these 



