B. Porifera incalcaria. II. Spiculispongiae. 3 



upper part spongophare ) rising dome-like from a flat attached base (hypo- 

 pharect). The walls consist of 3 layers of tissue; a connective tissue being the 

 chief, middle layer, lined with epithelium on both sides. The large cavity of the 

 sac (paragaster) has one large opening (oscule) by means of which it commu- 

 nicates directly with the sea- water. The lining epithelium is flat (pinacocytes) ; 

 but the recesses in the wall, known as flagellated chambers, are lined by collared 

 cells (choanocytes) . These chambers communicate with the paragaster by means 

 of a wide mouth (apopyle), and with the exterior by a small pore (prosopyle). 

 This Rhagon doubtless is readily derivable from the Ascon ; but it never actually 

 passes through an Ascon stage. The successive stages, by which a Rhagon 

 passes into an adult sponge, have not been traced. But the investigations of 

 Schulze seem to prove that it consists in a folding of the spongophare. The inner 

 sinuses of the folds or lobes of the spongophare are continuous with the original 

 paragaster ; they form together the excurrent canals, and the outer sinuses form 

 the incurrent canals. The simplest case is that the flagellated chambers continue 

 to communicate directly with the paragaster through the apopyles. Thus several 

 chambers open directly into a common excurrent canal ; this chamber-system is 

 called eurypylous. Complications occur as the folds grow together, and a thin 

 investing membrane appears. In such investing membranes, which cover, tent- 

 like, the primitive (incurrent) folds or canals, the pores are often situated, not 

 however, homologous with the original pores of the Rhagon. The investing skin 

 soon becomes more consistent and may be called ectosorne, the rest of the sponge 

 choanosome. The ectosome is thus of secondary origin and the endoderm does 

 not enter into its composition. The incurrent canals and lacunae, which lie im- 

 mediately beneath the ectosome, are called subdermal cavities. It is not yet 

 known in which manner the ectosome arises. Sometimes it seems that the sub- 

 dermal cavities are fissures ; in such cases they are not lined by ectodermal epi- 

 thelium, but by endothelium. In Stelletta phrissens they seem to originate from 

 imaginations of the ectoderm. The mesoderm shows various modifications 

 and develops a large volume. Coinciding with this there are changes in the 

 chamber- system. The apopyles no longer lie at the same level as the sur- 

 face of the excurvent canal into which they open, but there is a longer or shorter 

 canaliculus (aphodus) between the chamber and the main excurrent canal. This 

 chamber-system is called aphodah<. If now also the communication between the 

 prosodus and the exterior is no more a direct one, but if also there canaliculi ap- 

 pear, the system is called diplodal. Changes in the canal-system are often 

 accompanied by modifications of the ectosome. In sponges with eurypylous cham- 

 bers the ectosome never attains any high degree of differentiation. It consists 

 there of an investing membrane, composed of a thin layer of mesodenn bounded 

 on both sides by ))ectodermal epithelium. The subdermal cavities are never com- 

 pletely differentiated from the incurrent canals ; they communicate directly with 

 a greater or smaller number of the flagellated chambers. In the simplest sponges 

 with an aphodal chamber-system, such as Myriastra, the ectosome does not mar- 

 kedly differ from that already described. But in Pilochrota, e.g., it exhibits a more 

 differentiated structure. The mesoderm extends inwards and in that way the 

 subdermal cavities become reduced. In the central portion of the ectosome fusi- 

 form contractile cells make their appearance; they increase in number and 

 strength, and finally form a thick felt. The ectosome is then generally differen- 

 tiated into two portions, an outer collenchymatous and an inner fibrous. Such 

 an ectosome is called cortex. In sponges with a well-developed cortex, the 

 intercortical canals (chonae) ware usually of a very definite characters The 

 sphincter which is found between endo- and ectochone, is to be considered as an 



