CALCAREOUS SPONGES. 705 



The next important advance in the history of the group was 

 the erection of the genus Leucosolenia by Bowerhank in 1866, 

 for certain sponges which we now inchide in the Homoctielidfe, in 

 addition to other genera no longer employed. From that time 

 forward numerous investigators studied the group, and manj- 

 new species and several new genera were described, but no really 

 serious attempt to deal with the question of the classification of 

 these sponges was made until the time of Haeckel, who in 1870 

 published his ' Prodromus,' and in 1872 his famous Monograph 

 of the group, with extremely detailed, though somewhat un- 

 successful descriptions of all the then known species, including 

 many which he described for the first time. Haeckel's so-called 

 " natural " system, with its three families of Ascones, Leucones, 

 and Sycones, based upon the type of canal system, and its twenty- 

 one genera based upon the types of spicules present, is so Avell 

 known, and has been so often criticised, that it needs no further 

 description by us, especially as it proved extremely artificial, and 

 expressed only to a very limited extent the true phylogeny of 

 the group. 



The scheme proposed by Polejaeff in 1883 was considerably 

 more successful, and his primary division of the gi'onp into 

 HoMOCCELA and Heterocoela has been ixiade the basis of almost 

 every classification since proposed. We are now beginning to 

 realise, however, that this division also is of a very arbitrary 

 chai'acter. 



The next scheme of classification we need notice is that of 

 Vosmaer, in Bronn's ' Klassen und Ordnungen des Thierreichs ' 

 [1887], which is almost identical with that of Polejaefl', with 

 the addition of the Pharetronida? as a fourth family of the 

 Heterocoela. 



In 1891 von Lendenfeld proposed a modification of Haeckel's 

 system, ei'ecting a fourth family, the Sylleibidje, intermediate in 

 canal-system between the Leucones and Sycones, and reducing 

 the number of genera in each family to two, according to the 

 presence or absence of oxea. This was undoubtedly a con- 

 siderable improvement upon Haeckel's system, but again it failed 

 to interpret the interrelationships of the members of the groiip 

 correctly, and it has since been almost entirely abandoned, though 

 certain spongologists, notably Breitfnss, retained it with but 

 little modification for a considerable time. 



During the years 1891-1893 there was published by Dendy 

 [1891 A, 1892 B, 1893 A] a scheme of classification based on 

 almost wholly different lines. Retaining PolejaefF's Orders 

 Homoccela and Heteroccela, and, like that author, including in 

 a. single genus, Leucosolenia, all the species of Homoccela, he 

 divided the Heteroccela into five families, whose difterentiating 

 characters were based far more on the structure and arrangement 

 of the skeleton than on the canal system ; and although this 

 system has not been accepted by all writers, yet we ourselves feel 

 that it embodies a moi'e natural arrangement of the group than 



