2l'4 "TER1!A NOVA' EXPEDITION. 



attachineiit, and these, coupled with tlie outer chamberlets of the free form, 

 constitute a basal layer sufficiently thick to mask entirely the rotaline arrange- 

 ment of the free form. It seems probable that an alternative method of growth 

 is for the young individual to settle direct as a primordial chamber on its host, 

 and then to construct a minute rotaline test, which, until masked by the super- 

 imposition of chamberlets, would be indistinguishable from a young sessile speci- 

 men of Truncatulina or Discorhina (Fig. 29). 



Adult " Sessile "' Stage. — Throughout these early stages the shell- wall is 

 very thin and delicate, exhibiting the characteristic tubulation of Pohjtrema very 

 distinctly. With advancmg growth many of the. specimens become very thick- 

 walled, owing to the deposition of shell-matter in solid layers, or as tubular 

 outgrowths from the surface. These outgrowths of shell-substance are usually of 

 a much deeper tint than the surrounding test. The continual formation of thin 

 investing layers of chambers leads to the formation of deeply sunken pits, more or 

 less circular in outline (Fig. 26). In many of these pits or " craters '"' the circular 

 floors are seen to be finely tubulated, suggesting, at first sight, an attribution 

 to Homotrenia, Hickson, instead of to Polytrema, but the persistent tubulation 

 of the high ridges surrounding these craters is characteristic of Polytrema, and 

 there can be no doubt but that these sunken circular patches represent the initial 

 stages of development of the " pillar-pores " marking the genus, and that their 

 origin is due to the spreading of a new stratum of protoplasm over the previously 

 formed shell in an alveolar layer instead of in a solid film. We have observed 

 similar specimens from other localities. No specimens of Hotnotrema were found 

 in either the " Terra Nova " or the Corsican material, and its distinctive characters 

 are such that it could not be overlooked. 



The occurrence of siliceous spicules inside the chambers has furnished nuiterial 

 for many controversies in the past. Carter (1870 and 1876) who had a highly 

 critical mind, and was also a Sponge specialist, and Schultze before him had no 

 doubt whatever in attributmg their occurrence to the inception of the spicules 

 by the organism, either as food or as " ingesta,'' which by accident have been 

 drawn in by the pseudopodia and have accumulated like the hairs forming the 

 " hair-ball "" in an ox's stomach. The N.Z. specimens are not so spiculiferous as 

 those from other localities which we have examined, Init an amount of this 

 material was worked out which satisfies us that the adventitious character- of 

 the spicules is beyond question. They are usually confined to the apertm'al arms 

 and the external layer of chamberlets, and they represent forms characteristic 

 of widely separated groups of Sponges, usually Monaxonid. Init occasionally 

 Tetractinellid. The curious sickle-shaped flesh-spicules characteristic of some 

 sponges have been identified. When the spicules are small they are often quite 

 perfect, but larger spicules are usually more or less broken. Diatoms, Radio- 

 laria, and other minute organisms are also found ingested in the chambers. 



