192 PORIFERA 



inflorescences. These types formed the basis of Haeckel's 

 famous classification. 1 It has, however, been concluded 2 that 

 the skeleton is a safer guide in taxonomy, at any rate foi 

 the smaller subdivisions ; and in modern classifications generc 

 with canal systems of the third type will be found distributee 

 among various families ; while in the Grantiidae, Ute an( 

 Leucandra stand side by side. This treatment implies a beliei 

 that the third type of canal system has been independently an( 

 repeatedly evolved within the Calcarea an example of a pheno- 

 menon, homoplasy, strikingly displayed throughout the group. 

 It is, remarkably enough, the case that all the canal systems 

 found in the remainder of the Porifera are more or less modifiec 

 forms of one or other of the second two types of canal systei 

 above described. 



The families Grantiidae, Heteropidae, and Amphoriscidae, all 

 possessing a dermal cortex, are distinguished as follows : Th( 

 Grantiidae by the absence of subdermal sagittal triradiate 

 spicules and of conspicuous subgastral quadriradiates ; the 

 Heteropidae by the presence of sagittal triradiates ; the 

 Amphoriscidae by the presence of conspicuous subgastral quadri- 

 radiates. 



Two families of Calcarea, possibly allied, remain for special 

 mention the Pharetronidae, a family rich in genera, and con- 

 taining almost all the fossil forms of the group, and the Astro- 

 scleridae. 



The Pharetronidae are with one, or perhaps two exceptions 

 fossil forms, having in common the arrangement of the spicules of 

 their main skeletal framework in fibres. The family is dividec 

 into two sub-families : 



I. Dialytinae. The spicules are not fused to one another ; 

 the exact mode of their union into fibres is unknown, but ai 

 organic cement may be present. 



Zelapia australis, a recent species, should probably be placed 

 here as the sole living representative. Dendy has shown 3 that 

 this remarkable species has a skeleton of the same fibrous character 

 as is found in typical Dialytinae, and that the triradiate spicules 

 in the fibres undergo a modification into the " tuning-fork " type 

 (Fig. 86, C), to enable them to be compacted into smooth fibres 



1 "Die Kalkschwamme, " 1871. 2 Dendy. loc. cit. p. 159. 



3 Quart. Journ. Micr. Sci. xxxvi. 1894, p. 127. 



