7 6 Introduction to ^[ nun a I Morplwlogy. 



verted into mother-cells, whose nuclei become sper- 

 matozoa, and rupturing the cell wall these flagellate 

 bodies are emitted, and fertilize the ova, which become, 

 by segmentation, first morulae, then ciliated germs, 

 with a bilaminar membrane surrounding a central 

 cavity (planulaej, whose pole becomes thinned and 

 perforated by absorption (Ifacckcl: Mctschnikoff &&- 

 scribes the cavity in Sycon ciliatum as formed by 

 imagination). Sexual reproduction has been detected 

 in Tethya, Reniera, many Calcispongia, &c. 



2nd. Asexual reproduction may occur by division, 

 natural or artificial, or by budding, either continuously, 

 thus forming colonies, or discontinuously, by the for- 

 mation of detached gemmules. In the freshwater 

 Spongilla these are either ciliated plastides or swarm- 

 spores, or else rounded masses found in the older 

 parts of the animal's body, consisting of a central 

 cluster of soft cells surrounded by a globular coriaceous 

 capsule pierced by an opening, and outside which is 

 a crust containing radial, dentated birotulate spicules 

 (Amphidiscs). These "winter eggs" grow in spring 

 by processes protruded through the infundibulum, or 

 opening. 



Sponges are divisible into eight orders: 



Order i. llalisarcia a doubtful group, including soft, 

 skeletonless forms, probably immature states of other Sp 



Order 2. Ceraospongia Sponges with a skeleton of 

 Spongiolin fibres, 0.07-0.004 mm. in diameter, making an 

 irregular horny net-work. These may be similar, homo- 

 geneous, structureless (when young), or consisting of con- 

 centric laminae (Spongia, Ditela) ; or with a differentiated axis 

 (Verongia, Aplysina) ; or an axis of sand grains and of the 

 spicules of other Sponges (Dysidea) : or of special spicules, 

 either fully included (Chalina) or projecting on the surface 

 (Clathria) ; or else some fibres may be coarse and with a 



