REPORT ON THE MONAXONIDA. Iv 



The form of the spicules in both groups is exactly the same. Hence there is good reason for 

 supposing that the Chalininse have been derived from the Eenierinse by the development 

 of spongin at the expense of the spicules, and, moreover, they seem to be of polyphyletie 

 origin, descended from several distinct genera of Eenierine sponges. 



In the Heterorrhaphidfe we find examples of exactly the same thing. The genus 

 Gelliodes differs from the genus Gellius only in the greater development of horny fibre, 

 and the genus Toxochalina contains so much spongin in the fibre that it has hitherto 

 been placed amongst the Chalininse, although its spiculation shows it to be one of the 

 Heterorrhaphidae. 



Amongst the Desmacidonidfe we have abundant instances. We have species of the 

 genus Esjierella with little or no horny matter, and other species with a well-developed 

 horny skeleton, and precisely the same thing occurs in the genus Myxilla. Clathria and 

 Echinoclathria are other instances of genera of the Desmacidonidse in which there is 

 a very highly developed horny skeleton. 



In the Axinellidse the same facts may again be observed, and the Challenger dredgings 

 have brought to light a highly instructive example of an Axinellid sponge [Axinella 

 Jibrosa, nobis) possessed of a very strongly developed horny skeleton. 



We have abeady remai'ked the fact that the siliceous skeleton decreases in proportion 

 as the horny skeleton increases, until i;ltimately it disappears and leaves us face to face 

 with a horny sponge. No wonder then, considering their probable origin from many 

 distinct groups of siliceous sponges, that Polejaeff found such great difficulty in classifying 

 his Keratosa. 



We must mention in this jilace another very suggestive and important fact with 

 regard to spongin, and that is, that its degree of development depends upon locality. 

 Sponges with horny fibre are far and away more abundant in tropical or subtropical seas 

 than in temperate or frigid areas. When a sponge gets into a warm area it tends to 

 develop horny fibre. This is true at least of the Halichondrina amongst Monaxonida, 

 as is sufficiently shown by a glance at our tables of geographical distribution and the 

 description of those species obtained from the warmer areas. 



As regards the value of the minute anatomy and histology from a systematic point of 

 view we are hardly able as yet to form any conclusions, for our knowledge of this subject 

 is at present in its earliest infancy. We have in the last chapter given details on this 

 head which will, we hope, ultimately be of use in settling the question ; at present all 

 we can do is to collect information. So far as our researches yet go it seems that the 

 nature of the ectosome, i.e., whether it forms a fil^rous cortex or exists as a thin dermal 

 membrane or as a thick gelatinous layer, is likely to be of great systematic value. 

 Amongst the Halichondrina we know of only a single instance, the genus Phelloderma, 

 nobis, in which the ectosome forms a distinct, fibrous cortex, whilst amongst the Clavulina 

 a distinct, thick, fibrous cortex is abamst invariably present. The arrangement of the 



