PRINCIPAL GROUPS OF THE SPONGES. 



179 



Ascones of Haeckel, and includes the recent genus Leucosolenia 

 ( = Ascetta). No fossil forms belonging to this order are known 

 to occur. 



Order 2. Heterocgela. — In this order "flagellated chambers" 

 are present, and there is a canal-system of different form in the dif- 

 ferent families of the order. In the family of 

 the Syco?iidce ( = the Sycones of Haeckel) the 

 flagellated chambers are elongated, and con- 

 stitute a series of radial tubes which open di- 

 rectly into a central cloacal chamber. This 

 family includes such recent genera as Grantia 

 and Sycon, together with the Jurassic genus 

 Protosycon. This latter is the earliest repre- 

 sentative of the modern Calcisponges, and pos- 

 sesses a cylindrical or clavate body, with a long 

 tubular cloaca, and having the three-rayed or 

 four-rayed spicules so disposed as to form a 

 series of radial canals. 



In the family of the Leuconida ( = the Leu- 

 cones of Haeckel) the flagellated chambers are 

 mostly rounded, and are placed in communi- 

 cation with the cloacal chamber by exhalant 

 canals ; while the skeletal spicules are arranged 

 in no definite order. This family includes such 

 recent genera as Leuconia and Leucandra, de- 

 tached spicules of both of which have been 

 recognised in the Pliocene beds of St Erth, 

 Cornwall. Very closely allied to this family is 



the 2TOUD Of the Tekhonidce, Of Which nO foSSil careous Sponge, "enlarged 

 ° r . fifty times. (After Haec- 



torms are known. kei.) 



Lastly, we have the large and important 

 group of the Pharetronidce. (the Pharetrones of Zittel), all the 

 members of which are extinct. This group includes the largest 

 and most massive of all the Calcisponges, and its characters depart 

 in many important respects from those of all recent forms. The 

 Sponges included by Zittel under the name of Pharetrones are 

 variably shaped, simple or branched sponges, with thick walls, and 

 attached by their bases to foreign bodies. In some forms (e.g., in 

 Peronelld) the canal-system is not distinctly developed, but there is 

 usually a special system of irregular, branched water-canals. The 

 skeleton consists of calcareous spicules disposed to form a network 

 of solid anastomosing fibres (fig. 71, d), and there is a well-de- 

 veloped smooth or wrinkled dermal membrane. 



The earliest known types of the Pharetrofies appear in the De- 

 vonian rocks, and belong to the genus Peronella, which in later 



Fig. 70. — Ascetta prim- 

 ordial™, a simple Cal- 



