SPONGES. 71 



about 0"2 millim. across; the cladi are usually extended nearly at right angles to 

 the shaft ; they are short and each usually divides into two short, sharp-pointed 

 branches ; occasionally, however, the cladi are uubranched and sometimes they are 

 very irregular. 



(2.) Spined microxea (Plate V., fig. 1); very slender, straight or very slightly 

 curved, with spination minute but sharp and abundant and fairly uniform throughout 

 the length, excepting that there is frequently a constriction in the middle of the 

 spicule, in which the spination may be more or less wanting ; size about 0'02 millim. 

 by '00 133 miJlim. (excluding spines). These spicules occur very abundantly in the 

 dermal crust already mentioned and also scattered throughout the sponge. 



The ectosome forms a cortex about 0'27 millim. thick, composed of large oval 

 cystenchymatous cells with fibrous tissue between them, the fibres, for the most part 

 at any rate, lying parallel with the surface. The cystenchyme cells measure about 

 0'06 millim. in longer diameter and are also abundant in the choariosome. 



This species is obviously very closely related to the European Stceba plicata, on 

 the one hand, and to S. simplex, from the Gulf of Manaar, on the other. From the 

 former, which has been fully re-described by TOPSENT (14), it differs in its longer and 

 slenderer and more sharply-pointed microxea, and in the fact that the trisenes are 

 nearly all dichotriaenes. From the latter it differs in the smaller size of the trisenes 

 and the larger size of the microxea, and in the fact that the spines of the microxea 

 are not " most prominent towards the ends," though, as I have pointed out above, 

 they may be more or less absent from the middle of the spicule. It appears to be a 

 good deal more robust in growth than either species. It is quite possible, however, 

 that subsequent researches may make it possible to consider all three as mere 

 varieties of one species, but for the present it seems desirable that they should be 

 kept separate. 



TOPSENT describes the ectosome of Stceba plicata as being collenchymatous, but he 

 mentions and figures large " cellules spheruleuses," which are evidently closely similar 

 to the large cystenchyme cells of S. extensa. The partly fibrous cortex of the latter, 

 almost devoid of megascleres, may possibly afford another means of specific 

 distinction. 



In external appearance (in spirit at any rate) Stceba extensa bears such a close 

 resemblance to the thin encrusting form of Chondrilla australieHsis that it is difficult, 

 if not impossible, to distinguish the two without microscopical examination. 



RN. 1G7 (Station LXVL, off Mutwal Island, March 19, 1902, 10 to 35 fathoms). 



FAMILY : STELLETTID^E. 

 Astrophora witli long-shafted trisenes, without calthrops and without sterrasters. 



Myriastra, SOLLAS. 

 Microsclere a euaster of one form only. Ectosome not a cortex. 



