OP THE SPONGIADiE. 81 



CELLULAR TISSUE. 



The cellular structures in the Spongiadse are few and 

 very simple in form. We find no series of conjoined cells 

 in the body of the sponge, as in vegetable tissues. The 

 only forms in which true cellular structures occur in the 

 bodies of sponges, are those of detached spherical molecular 

 cells, and of discoid or lenticular nucleated cells. The first 

 forms are found in abundance on the fibres of many species 

 of the true sponges, and are believed by Dr. Johnston to 

 be the reproductive organs of that genus. They are very 

 minute, not exceeding 1^550 ™ch in diameter. They are 

 pellucid, and aflbrd no indications of a nucleus, either single 

 or multigranulate (Eigs. 315, 316, Plate XXII). 



Imbedded in the sarcodous stratum on the interstitial 

 membranes in many of the Halichondroid tribes of sponges, 

 we frequently find numerous compressed circular cells. In 

 the greater number of cases they are so translucent as to 

 readily escape observation even with a tolerably high power; 

 but in other species, as in Ecionemia acervus, Bowerbank, 

 MS., a new genus of sponges from the South Seas, in the 

 collection of the Royal College of Surgeons, and in Hali- 

 chondria nigricans, Bowerbank, a British species, these 

 tissues are developed in a more than usually distinct 

 condition. 



In the first-named sponge they are thickly dispersed on 

 the surfaces of the interstitial membranes, but without any 

 approach to order or arrangement. They are decidedly 

 lenticular in form, with a well-defined transparent nucleus, 

 which varied in size from about one fourth to three fourths 

 the diameter of the cell in which it was contained. The 

 cells varied considerably in size : the largest I could find 

 was 5^0 inch in diameter, and one of the smallest 15,500 inch ; 

 but the greater number were about ^ inch in diameter 

 (Kg. 281, Plate XVI). In Halichondria nigricans they do 

 not appear to be quite so convex as in Ecionemia acervus, 

 nor are they so numerous as in that species, but they are 



