REPORT ON THE RADIOLARIA. Ivii 



surrounds the oral half iu the form of a voluminous concavo-convex cajj, hiding the 

 astropyle at its basal pole so completely that the latter is rarely visible until the 

 phjBodium has been removed (Pis. 99-104; PL 115, fig. 8; PI. 123, &c.). The 

 central capsule is generally almost completely embedded in the phseodium, so that only 

 its aboral pole (with the two parapylse in the Tripylea) projects. In the Pheeo- 

 g r o m i a, in which the lattice-shell possesses a special opening and the central capsule 

 lies excentrically in the aboral portion of its interior, the phieodium occupies the oral 

 aspect, between the capsule and the aperture (Pis. 99, 100, 118-120, &c.). In the 

 peculiar family Coelographida (Pis. 126-128) a special receptacle (galea with its 

 rhinocanna) for the phseodium is developed outside the bivalve shell, within which the 

 central capsule lies. The proboscis, which in all Ph^odaria arises from the centre of 

 the astropyle, lies in the vertical axis of the phasodium, and is entirely surrounded by 

 it. The volume of the phseodium in the majority of the Ph.eodaria may be said to 

 be about as great as that of the central capsule, although in some species it is consider- 

 ably larger. Its colour is always dark, usually between green and brown, commonly 

 olive-green or blackish-brown, rarely reddish-brown or black. The phseodeUse or 

 pigment-granules which make up the greater part of the phseodium (see note B) are 

 irregular in form and unequal in size and show no definite structure ; usually they are 

 spherical or ellipsoidal, and exhibit fine parallel striae which run transversely or obliquely 

 (PL 101, fig. 3, 6, 10 ; PL 103, fig. 1, &c.). Between the larger granules is 

 usually found a thick dust-like mass of innumerable very small grains. The physio- 

 logical significance of this peculiar phseodium is still unknown, but is probably consider- 

 able, judging from its large size and especially from its constant topographical relation 

 to the astropyle; the latter consideration would lead to the supposition that it plays an 

 important part in the nutrition and metastasis of the Ph.eodaria (see note C). 



A. The phpeodium of Aulacantha, Thalassoplancta, and Cwlodendrum was first described in 1862, 

 iu my Monograph, as an excentric extracapsular mass of pigment of blackish-brown or olive-green 

 colour (pp. 87, 262, 264, 361, Taf. ii. iii. xxxii.). Since then John Murray, who investigated many 

 living Ph^odaeia during the Challenger expedition, has shown its general distribution in this 

 legion (Proc. Eoy. Soc. Lond., vol xxiv. p. 536, 1876). From the constancy of its presence I gave 

 the legion the name Ph^odaria in 1879 (L. K 34). 



B. With regard to the special composition of the phseodium and the constitution of the 

 phaeodeUse, see the general description of the Ph^eodaria, pp. 1533-1537. 



C. Perhaps the pha-odells are to some extent symbiontes with the Ph.eodaria ; the xantheUte 

 present in most other Radiolaria are absent in this legion. 



90. The Extracapsular XanthellcB. — Xanthellee or Zooxanthellse, symbiotic 

 " yellow cells," are very commonly found in the extracapsulum of the Eadiolaria, 

 especially in many Spumellaria and Nassellaria ; whilst in the Acantharia similar 

 yellow cells usually only occur within the central capsule, and in the Ph^odaria their 



(ZOOL. OHALL. EXP.— PART XL.— 1886.) • Er A 



