344> Dr Grants Observations on the 



At other times, we observe the papillae so slightly and gradually 

 raised, as to produce only an undulated smooth surface ; this is 

 frequently the case where it is much exposed to the direct force 

 of the waves or currents, or where it envelopes the roots and 

 stems of marine plants. It is more probable that these varieties 

 of form depend on situation, or other accidental circumstances, 

 than that they are perpetuated by generation ; for we sometimes 

 find these different kinds of surface on the same specimen. 

 Where the papilla has a regular conical form, the margins of its 

 fecal orifice are circular, thin and translucent, but in the ridged 

 portions the fecal orifices are often very wide, with abrupt irre- 

 gular broken margins ; when we look down into one of these wide 

 orifices, we distinctly perceive, on every side, the terminations of 

 the internal canals, which unite and widen as they approach the 

 orifice, and open at such an angle, as to throw their currents 

 nearly in the direction of the main stream. This part of the 

 anatomy is beautifully exhibited, by cutting a living papilla per- 

 pendicularly into several sections. The ova, in this species, be- 

 gin to appear in the deeper parts of its substance between the 

 canals, near the end of February ; they are in full maturity in 

 May, and not a trace of them is discernible in its texture at the 

 end of June. The surface of the papillaris is often quite green, 

 although its whole texture within is filled with ova of a lively 

 yellow colour ; and sometimes we find a specimen, some parts of 

 which are yellow, and others green, although the whole interior 

 is equally filled with ova. In dried specimens of the papillaris 

 the internal texture may be rendered as soft as the finest down, 

 and of a pure white colour, by careful and repeated maceration 

 in hot water ; in this condition, the part covering the whole outer 

 surface appears like a thin white calcareous crust, lying loosely 

 over the downy texture, and everywhere closely perforated with 

 regular minute angular pores, which are now rendered more dis- 

 tinct, by the removal of their soft parts, and by the contraction 

 of the loose fasciculi which surround them This thin white shin- 

 ing porous covering contains no lime, but appears to have been 

 sometimes taken by authors for a calcareous crust, both in sponges 

 and in other allied genera. 



When a very thin portion, cut from the surface of the recent 

 papillaris^ is viewed on a plate of glass under the microscope, we 



