144 SPONGES 



as the elastic fibrillae described by him in Reniem. The question cannot 

 at present be decided. 



Phylogeny of Keratose Skeletons. In dealing with the Monaxonida, the 

 evolution of the pure spongin fibre, by gradual increase of the spongin and 

 atrophy of the spicules in the skeletal fibres of that group, has already 

 been traced (see above, p. 139). It is highly probable not only that most 

 Keratose skeletons have so originated, but that the evolution of spongin 

 fibres has taken place in this way more than once in different families of 

 Monaxonida independently. On the other hand, it is not improbable that 

 the dendritic fibres of the Aplysillidae may have originated in a different 

 way, which, however, it is not possible to indicate satisfactorily at present. 



After loss of the spicules, many sponges have acquired the habit of 

 taking up foreign bodies into their fibres, a habit which reaches its 

 extreme in the arenaceous Sponyeliidae. Should some of these forms 

 prove to be really devoid of spongin, an interesting speculation is opened 

 up as to how far such a condition is the culminating point in an evolution 

 which proceeds by diminution and ultimate loss of spongin ; or whether 

 it is a more primitive state of things, spongin never having been present. 



Histology. As has been already remarked, the Demospongiae attain 

 to a higher degree of histological differentiation than either the Calcarea 

 or the Hexactinellida ; while in the two latter classes we can scarcely 

 recognise more than the six categories of cells indicated by Roman numerals 

 in the table given above (p. 62), in the Demospongiae each of these cell- 

 species may be further differentiated into the several cell-varieties indicated 

 in our table by Arabic numerals. Since these many forms of -cells have 

 already been fully described above, we need not further discuss them 

 here. It should, however, be pointed out that our knowledge of the 

 histology of Demospongiae is still in a very backward condition, and that 

 it is extremely difficult to refer with certainty the numerous forms of cells 

 to their proper position in a phylogenetic classification of the histological 

 elements. Amongst the authors who have especially contributed to our 

 knowledge of these questions in recent years, Topsent deserves especial 

 mention as having been the first to show the connection of the myocytes 

 and the epithelium, and also as having demonstrated the existence in all 

 Demospongiae of cellules spheruleuses. The latter are almost certainly 

 homologous, as pointed out above, with the porocytes of Calcarea, although 

 their connection with pores has not yet been demonstrated and may not 

 exist. In support of this conclusion, reference may be made to the recent 

 investigations of Loisel, above described. 



Embryology. The structure and metamorphosis of the larvae of 

 Demospongiae has been dealt with above at sufficient length. "We may 

 refer, however, to two points of interest. The first is the striking fact 

 that in the whole group of Tetractinellida, comprising as it does many 

 abundant shore forrns, no larvae are as yet known. The second is the 

 occurrence, in the larvae of Monaxonida, of diagnostic characters corre- 

 sponding to the systematic position of the adult sponges (Maas). Thus 

 in Haploscleridae the larva has a pigmented ring at the posterior pole, the 

 pigment being chiefly lodged in a circle of larger flagellated cells, which 

 bear flagella of a special type, and mark the posterior limit of the 



