Bihli'ofjntphical Notice. 421 



Darwinelh, ¥. Miillor, which possess a skoleton maiuly of homy 

 spicules detached from each other and irregularly scattered in the 

 mesoderm of the sponge. Only two species are as yet known : in 

 the tirst described, D. aiirtit, the spicules have from three to eight 

 rays ; some of them resemble the four-rayed or Calthrops spicules 

 of siliceous Tctractiiicllid sponges, whilst others approach in form 

 the six-rayed spicules of Hexactinellids. In the other 8j)ccies, D. 

 (lustrdlii'iisis. Carter, the large majority of the spicules have only 

 three rays in one plane, and thus singularly resemble in form the 

 three-rayed spicules so common in C'alcisponges, The author con- 

 cludes that these varied forms of horny spicules in DarivinelJa are 

 directly derived from the siliceous spicules of the Ilexactinellida in 

 which the silica has been replaced by spongin ; but there seems 

 very little warrant for supposing that spicules so far removed from 

 the Hexactiuellid type as the Calthrops and three-rayed forms can 

 ever have been derived from normal six-rayed Hexactinellid 

 spicules ; if they have been derived from siliceous sponges at all, 

 they arc more nearly related in form to retractiucllid spicules. 



Yet further, Dr. Lendenl'eld states that the substitution of spongin 

 for silica in these horny spicules has been brought about to meet) 

 the " exigencies of changed circumstances icsulting from a migra- 

 tion from the siliciferous depths of the ocean to shallower water, 

 ■where the amount of silica contained in solution in the water is not 

 so great "' ! It may be asked if there is any reason for believing 

 that the water of the ocean at great depths contains more silica than 

 in shallower areas ? Judging from the abundance of recent siliceous 

 sponges in shallow and moderate depths, and from their enormous 

 development under similar conditions in past ages, there is no ground 

 whatever for supposing that the spicules of siliceous sponges would 

 be at all likely to undergo substitution of spongin for silica through 

 a comparative scarcity of this mineral in shallow water. 



The author justifies the inclusion in Horny Sj)onges of such 

 genera as HaJisarca and Bajulus, in which there is no horny skele- 

 ton whatever, on the ground that tliey are rudimentary horny 

 sponges ; on the other hand, Schulzc considers these forms as rudi- 

 mentary Hexactinellids ! 



A total number of 248 distinct species and varieties are described 

 in this work, of which no fewer than 258, or 74 percent , are found 

 in the Australian seas, whilst 179 species are limited to this region. 

 Horny sponges are distinctively inhabitants of shallow water, the 

 greater number occurring at depths between 20 and 50 metres, and 

 the greatest depth at which they have been met with is 750 metres. 

 They also flourish most in warm seas. 



In the synthetical part of the volume the general results deduced 

 from the empirical descriiitious are discussed in a series of chapters 

 in which the structure, classification, and systematic positions of 

 sponges generally are treated. We can here only touch upon a few 

 salient points, and one of these is the statement that the canal- 

 system is the most important organ in sponges, and that it should 

 principally be taken into account in classifying them. But is it not 

 the fact that an essentially similar caual-sysiem is present in many 



