1893.] MICROSCOPICAL JOURNAL. 345 



flagellated cells (choanncytes) in wliicli a nucleus, a nucleolus, 

 and one or more contractile vacuoles may be seen. It is worthy 

 of remark that the flagellated cluunbers and the choanocytes 

 become smaller as we advance to a more complicated canal sys- 

 tem, thus in the simple Sycon the chambers are the largest; they 

 become smaller at each step, until in the dii»lodal Rhagon they 

 are the smallest. 



In most sponges the mesoderm is largely developed ; in its 

 commonest and simplest form it consists of a clear, colourless, 

 gelatinous matrix, in which irregular branching stellate cells or 

 connective tissue corpuscules are embedded. The tissue is 

 called collenchyme and the cells collencytes. In the higher 

 sponges it consists of small, polygonal, granular cells, cemented 

 together by a small quantity of structureless jelly ; this is called 

 the sarcenchyme. Scattered amongst the collenchyme are wan- 

 dering amoeboid cells, archseocytes, some serving as carriers of 

 food, others acting as scavengers, carrying awa)' all useless or 

 irritant foreign matter; some of these cells become converted 

 nto sexual products. Another tissue, cystenchyme, is found 

 in many of the liigher sponges, especially in TetractinelUdu. It 

 consists of closely adjacent, large, oval cells with thin walls and 

 fluid contents; near the centre of the cell is the nucleus and its 

 nucleolus, it forms a layer just below the skin of such sponges 

 as Pachymatisma, and is supposed to be a fatty tissue, for, in 

 teasing out a piece of the cortex in water, oily globules are set 

 free. This oil must be soluble in alcohol, for sponges which 

 have been kept in spirit show no trace of it. Chondrenchyme is 

 a tissue which resembles cartilage ; it is found in the Corticidae. 

 Connective tissue cell, desmacytes, are present in most sponges. 

 Being closely packed together they sometimes form a kind of 

 felted sheet, which in the ectosome of some sponges acquires a 

 considerable thickness, often constituting the greater part of the 

 cortex. The sterreasters of Pachymatisma are united together 

 by these cells. Contractile fibre cells, myocytes, occur in all 

 higher sponges. These are fine, granular, fusiform cells, with 

 long filiform terminations. In most sponges the incurrent and 

 excurrent canals are constricted at intervals by a muscular 

 sphincter produced by the excessive development of myocytes. 

 Their functions can be easily demonstrated, for irritation of the 

 margin of the osculum of such sponge as Pachymatisma is fol- 



