A. General. 1. Histology. 3 



- The epithelium lining the gastral cavity and exhalant canals resembles minu- 

 tely the ectodermal epithelium ; though in the cloaca of a Leucandra swollen vacuo- 

 lated cells were observed. In Grantia labyrinthica subderrnal gland-cells are more 

 plentiful on the gastral than on the dermal surface , this condition is rare. Vesi- 

 cular cells occur under the gastral epithelium of Ute syconoides. - - Prosopyles 

 are intercellular , not intracellular [cf. infra p 7 Maas] ; they are simply gaps 

 between cells ; even if perforated prosopyle cells do occur, it is not the usual con- 

 dition. Very distinct membranous diaphragms, of concentrically arranged 

 spindle-shaped cells, separate the flagellated chambers from the exhalant system 

 in most Heterocoela: ectoderni and endoderm may give rise to strikingly similar 

 muscular structures [cf. Bericht f. 1892 Porif. p 11 Minchin( 4 ), but cf. also infra 

 p 6 Maas]. - The existence of Soil as 's membrane may be only a transitory 

 condition . . . That it does exist in many cases is beyond dispute*. The peculiar 

 contraction of certain flagellate chambers in G. labyrinthica appears to be normal, 

 and not due to the action of reagents; discussed in connection with Bidder's 

 j)colunin-and-plinth hypothesis the condition is perhaps to be regarded not so 

 much a process of death as of rejuvenescence. - The mesoderm is usually 

 very sparingly developed in the walls of the flagellated chambers. It is probable 

 that no truly mesodermal muscular cells have yet been observed, and that the 

 supposed nervous cellsa are subdermal gland-cells. Stellate mesoderm 

 cells appear capable of becoming amoeboid. Glandular mesoderm-cells are of two 

 kinds, the subdermal gland-cells described above, and the calcoblasts. Spi- 

 cules are probably generated in primary calcoblasts, or mother-cells: while 

 secondary calcoblasts apply themselves to such spicules and increase their thick- 

 ness ; hence Minchin in Leucosolenia found 4 calcoblasts to a spicule. In large 

 spicules uniform increase must be explained by amoeboid movement of the calco- 

 blast. Ova: - - D.'s hypothesis [cf. Bericht f. 1S91 Porif. p 6] is confirmed that 

 the ova before fertilization migrate through the epithelium of the inhalant canals, 

 and hang freely from its surf ace. 



Topsent (*) takes Desmacidon fruticosus , Dendoryx incrustans , D. reses , and 

 Reniera viscosa , as types of mucous sponges, secreting continuously from their 

 surfaces; JR. indistincta is a typical viscous sponge, adhering to dissecting in- 

 struments like a glue ; Chalineae and Ectyoninae are mostly elastic, each frag- 

 ment tending to recover its form. These properties mainly depend on the 

 spherule- bearing cells (cellules sphe"ruleuses) which exist in all sponges, serving 

 as glands, reserves of nutrition, or organs of support: they may unite by fine 

 pseudopodia with the epithelia , and during most of their existence store up in 

 ssphe'rules dissociables reserve-substances, carbohydrates, mineral salts, various 

 lipochromes, and bodies yet unanalyzed. True mucus is secreted by Desm. and 

 Dend., the vesicular cells staining characteristically with aniline dyes. In Bubaris, 

 Axinella, and Raspailia, the spherule -cells are at once glands and nutritive 

 reserves ; the sponges are so viscous that Inachus and Pisa rub themselves against 

 them in order to be covered with adhering fragments (bourgeons) . In Reniera 

 indistincta the spherules contain a carbohydrate , besides the glue which im- 

 pregnates all the outer parts of the body. In the elastic Acervochalina the 

 spherule-cells have a supporting (conjonctif) function , and are disposed in com- 

 pact branching threads which anastomose in a continuous network. In the 

 Chalineae a small elastic ligament is found in the cell , colouring brightly with 

 aniline stains and taking part in the formation of the fibrils ; it has certainly not 

 a nuclear origin , though the nucleus is not found where it is present ; probably 

 the cell loses its individuality when the ligament is formed. Among Ectyoninae 

 large interlaced cells , with elongated or distorted spherules , are found in the 



c* 



