REPORT ON THE MONA.XONIDA. 



xU 



In short, all that is necessary iu order to bring about such a localisation of the pores is 

 that the subdermal cavities should be arranged at some little distance from one another 

 and have a number of pores leading into each. 



(3) Pores localised in areas owing directly to the hahit of the sponge. — We have only 

 one example to adduce belonging to this class, although very likely future researches will 

 show such a condition to be not uncommon. The example in question is a deep-sea sponge, 

 Tedania actiniiformis, nobis, obtained at Station 299 (off Valparaiso), at a depth of 2160 

 fathoms and on a bottom of blue mud. The external form of the sponge will be best 

 understood from the figure (PI. XL fig. 2 ; cf. also p. 55) ; it is attached to a piece of 

 rock. At a short distance below the flattened top there is a definite narrow zone 

 (PI. XI. fig. 2, p.z., and fig. 2a) of pores encircling the sponge. In this zone the pores 

 are so abundant as to reduce the dermal membrane to a mere network. Judging from 

 the general appearance of the sponge and the nature of the bottom upon which it lives, 

 we have little hesitation in saying that in life it was buried in the mud up to within a 

 short distance of the top, and hence the pores, in order that clean water might gain access 

 to them, became confined to that portion of the 

 surface which was above the mud, namely, a narrow 

 zone immediately below the flattened top. The genus 

 Tedania is characteristically an inhabitant of shallow 

 water, and in no other species do we find an arrange- 

 ment of the pores similar to that which characterises 

 this deep-sea form. There can be no doubt that in 

 this case the arrangement of the pores is dependent 

 directly upon the habit of the sponge, and not, as in 

 most cases, upon the arrangement of the dermal 

 skeleton or of the subdermal cavities. 



Before leaving the question of the pores we must 

 consider briefly the condition of flabellate sponges 

 in this respect. It is an almost invariable rule, that 

 in flabellate sponges the pores are to be found on one 

 surface and the oscula on the other. Thus in 



Phakellia ventilabrum, var. connexiva (PI. XXXV. figs. 3, 3a ; PL XLIX. fig. 3), and 

 Phakelliajlahellata, nobis (PI. XXXIV. figs. 2, 3, 3a), this arrangement is very well illus- 

 trated, and the same condition occurs in Myxilla frondosa, nobis (PI. XXVI. figs. 1, la); 

 and Gellius Jlabelliformis, nobis (PI. XXVI. figs. 5, 5a). Again in that very remarkable 

 sponge Esperiopsis challengeri (PI. XVIII.) the pores occur only on the concave surfaces 

 of the lamellae (PI. XVIII. fig. 4), while the oscula are all on the convex surfaces. 



By far the most remarkable instance of this kind is, however, afibrded by a boring 

 (zooL. CHALL. EXP. — PAET Lix. — 1887.) Nnn / 



Fig. XI. — Climia dissimilis. Vertical section of a 

 decalcified .specimen ; 0,0, osciila ; pa, pore-areas ; 

 c, ectosome ; ic, iuhaleut canals ; ec, exhalent 

 canals, x 4. 



