52 SPONGES 



section cutting, have become artificially separated from their spicules. 

 In the Myxospongiae, on the other hand, the parenchyma consists 

 entirely of connective tissue cells, none of which secrete spicules. 



The connective tissue cells or collencytes (Sollas) are marked out 

 by their clear protoplasm, free as a rule from coarse granules, and 

 by their fine thread-like pseudopodial processes (Fig. 49, a). In 

 both respects they usually stand in sharp contrast with the wander- 

 ing cells or amoebocytes, abundant, as a rule, in all parts of the 

 parenchyma, which in their more ordinary form are remarkable for 

 their very granular protoplasm and thick lobose pseudopodia, giving 

 the cell a form best compared to that of a potato. The collencytes 

 have been observed during life to be actively amoeboid, sending 

 out their thread-like pseudopodia and withdrawing them again. 

 The pseudopodia of two neighbouring cells may come into contact 

 and fuse temporarily. These changes of form may be accompanied 

 also by changes in the position of the cell as a whole (Schulze, 1877, 

 p. 16). As a rule each collencyte has several processes, but in other 

 cases the number may be reduced to two, giving the cell a more or 

 less elongate, bipolar form. Hence the connective tissue corpuscles 

 may be distinguished as stellate and fusiform, the distinction being 

 in most cases merely a temporary one, correlated perhaps with a 

 particular position. By further specialisation, however, of one of 

 these two forms of cells, and the acquisition by it of a definite 

 form and characters, certain classes of tissue elements become 

 marked out. Thus in most Demospongiae there are found special 

 fibre cells or desmacytes (Sollas ; Fig. 49, d), derived doubtless from 

 bipolar collencytes, and furnishing the elements which bind the 

 spicules together into sheaves and fibres to form a continuous skeletal 

 framework or a special fibrous cortex. In other cases, again, the 

 collencytes probably in the first instance those of the stellate 

 variety acquire a vesicular structure resembling to some extent 

 the vesicular connective tissue found in many invertebrates. Such 

 cells are termed " cystencytes " by Sollas, and the tissue composed 

 of them, " cystenchyme " (Fig. 49, c). 



According to the nature of either the cells or the ground substance of 

 the skeletogenous stratum, the body parenchyma may differ greatly both 

 as regards histological characters and consistence in different cases. 

 Sollas has distinguished a number of well-marked types of parenchyma 

 by appropriate terms : collencliyma, where the ground substance is abund- 

 ant, clear, and colourless ; sarcenchyma, where, on the contrary, the ground 

 substance is relatively less abundant and granular ; cliondrenchyma (Fig. 49, 

 &), where the ground substance is dense and the parenchyma of cartilaginous 

 appearance ; and finally, cystenchyma, which has been mentioned above. 



There remain finally for mention those elements of the dermal 

 layer which secrete the spicules. The scleroblasts when separate 



