316 



NATURAL HISTORY. 



the Sponge, and line the incurrent canals throughout ; the margins of the cells are 

 usually invisible, but can be readily developed by treatment with nitrate of silver. The 

 endoderm, or inner layer, lines the excurrent canals, and has the same structure as the ectoderm, 

 except in the flagellated chambers, i.e., the expanded ends of the smallest excurrent canals, where it 

 forms a single layer of flagellated cells (Fig. 6, en). These consist of a 

 spherical body of protoplasm, granular within, but firmer and clearer exter- 

 nally ; containing a nucleus, and one or more contractile vesicles ; one end is 

 seated on the wall of the chamber, the other is prolonged into its cavity as a 

 long neck of clearer hyaline protoplasm ; around its margin the end of the 

 neck extends into an immeasurably thin cylindrical or conical collar, while 

 from its centre is produced a long slender flagellum. 



The tissue between the two preceding layers is the mesoderm (Fig. 4) ; 

 it consists chiefly of a clear, greyish, jelly-like matrix containing irregularly 

 stellate granular nucleated corpuscles, united by branching processes into 

 an irregular network. In the neighbourhood of the flagellated chambers the 

 definite outline of the corpuscles disappears, and they merge together, 

 crowding the matrix with minute granules, amidst which the nucleus 

 remains unchanged. This gelatinous connective tissue is very similar to 

 that forming the disc of Jelly-fish ; it originates in cells, which first become 

 Fig. 5. FLAGELLATE COL- confluent, as about the flagellated chambers, and then change about their 

 LARED INFUSORIAN FEED- con fl uen t margins into the jelly-like matrix, their central part, with the 



ING, x 2,000 dia. (After . 



Savilie Kent.) nucleus, remaining as the stellate corpuscle, in certain places, as around 



The thecuTrV^i"ndure e d1)Tt c i"rota- the oscular openings, and in the circular diaphragms, which at intervals 



tory motion of the flagellum (JZ). . . . 1,1 



t>, food vacuoie ; n, nucleus ; c, con- constrict the main, water canals, the corpuscles present a fusiform shape, 



tracting vesicle. * 



acquire more or less distinct walls, and serve the function of muscle 



fibres. They present the same shape and appearances in other places, as parallel to the skeletal 

 fibres, and directed lengthwise in the walls of the main canals but here their function is that 

 of fibrous connective tissue. Besides these cells, which, though 

 contractile, are not locomotive, there are other amrebiform cells 

 which wander in the tissue, and frequently contain large granules 

 looking like fat or starch, serving no doubt as food reserves. 



The skeleton, which is a product of the mesoderm, consists 

 of a network of spongin fibres (Fig. 1), the substance of which 

 in chemical composition most closely resembles silk, both 

 compounds being regarded by chemists as horny matter. 

 The fibres may be distinguished as chief fibres and connecting 

 fibres ; the former, radiately arranged, project at right angles 

 to the Sponge-surface ; the latter form a network transversely 

 uniting the chief fibres together. Both have the same essential 

 structure, consisting of a thick, transparent, concentrically- 

 layered wall, and a soft granular axial thread ; but the larger 

 chief fibres contain in addition foreign particles, such as sand- 

 grains, sponge-spicules, and fragments of shell. They are 

 formed as a secretion by modified cells of the mesoderm ; and 

 the chief fibres obtain their included particles by embedding 

 at their soft terminations, the foreign material which lies plentifully strewn over the skin. 



Reproduction (Fig. 1). The ova and spermatozoa are found in the mesoderm. The former com- 

 mence existence as cells remarkably similar to the amcebiform corpuscles of the connective tissue, 

 being chiefly distinguished by their large bladder-like nucleus and its large round nucleolus ; as they 

 increase in size yelk-granules make their appearance, and at length the egg assumes a regular ovoid 

 form. It is noteworthy that the eggs in Euspongia are not, as in other Sponges, scattered irregularly 

 through the mesoderm, but occur in groups of ten to twenty in number near the large excurrent 

 canals embedded in a matrix of connective tissue, which is more or less separated from the rest of the 



6. ECTODERM, AND DIFFERENT FORMS 



OF ENDODERMIC CELLS FROM SCYCANURA 



RAPHANUS. (Ajter ScTmlze.) 



A, ec, ectoderm ; B, en, endoderm cells, x 500. 



