526 ON A COLLECTION FROM CHRISTMAS ISLAND. [June 23, 



The exhalant canal-system also consists of a series of more or less 

 lacunar spaces, opening into one aaother, and finally discharging on to 

 the surface through the oscula; their ultimate ramifications are of 

 comparatively large size, and are readily distinguished from the 

 ultimate inhalant lacunie by two important characters : (i.) they are 

 not subdivided by strands of mesodermal tissue ; (ii.) they are very 

 definitely bounded, and are surrounded by the flagellated chambers. 



The flagellated chambers, clustered around the exhalant lacunae, 

 open directly into the latter by means of vpide mouths, as shown in 

 the accompanying vFoodcut. There are no cameral canaliculi. The 

 proximal portion of each flagellated chamber appears, in the present 

 condition of the sponge, to project freely into the lumen of the 

 inhalant lacuna, in sucli a manner that it would be completely 

 immersed in the incurrent stream of water ; this appearance, however, 

 is probably in part due to the shrinking away of the surrounding 

 tissues owing to the action of the spirit in which the specimen was 

 preserved. In form the chambers are subspherical, and they are 

 very small, measuring only about 0-02 millim. in diameter. 



It is important to notice that the canal-system thus described 

 agrees essentially with that of the few other genera of Halichondrina 

 whose canal-system is as yet known to us. Minor differences, which 

 are likely to be of considerable importance for systematic purposes, 

 certainly exist in the arrangement of the canal-systems of these 

 different genera ; thus in the species under consideration the 

 structure and arrangement of the ultimate inhalant lacunae would 

 appear to be decidedly characteristic, possibly even affording a 

 character of generic importance, and that in a genus where such 

 characters are greatly needed ; but in all the Halichondrina the 

 fundamental type of canal-system appears to be the same — i. e., 

 according to Vosmaer's third type \ 



The iundamental agreement of the canal-system of Pachychalina 

 spinosissima with that of Halichondria panicea, a species which I 

 have also had the opportunity of studying carefully with well- 

 preserved material, and its close resemblance even in certain minor 

 details, may perhaps be regarded as an argument (though only of a 

 very general character) in favour of the view that the Chalininee 

 are very intimately related to the Renierince, and of uniting these 

 two groups as two subfamilies of the same family {HomovrhaphidcB, 

 Ridley and Dendy '). 



EXPLANATION OF PLATE XLIV. 



Fig. \. Pachychalina spviosissima, seen from the upper surface ; natural size. 



2. Portion of a sui-face-section, showing the arrangement of the pores 



and the dermal skeleton. 



3. Oxea. 



' Further details concei-ning the arrangement of tlie canal-system in tlie 

 Halichondrina are given by Mr. Ridley and myself in our Report on the Mon- 

 asonida dredged by H.M.S. ' Challenger.' 



^ Ann. & Mag. Nat. Hist. ser. 5, vol. xyiii. p. 32G ; and Report on the 

 Monaxonida dredged by H.M.S. ' Challenger,' p. 1, &c. 



