1887.] NATURAL SCIENCES OF PHILADELPHIA. 173 



when few in number they are smooth. Gemmula3 almost entirely 

 naked, Avithout the external air-chamber-layers, and with very few 

 spicules. In other cases they are covered to a greater or less depth 

 with a layer of minute cells filled with air. At times this layer is 

 surrounded with a distinct horny membrane, although it is often 

 wanting. In the air-chamber-layer are imbedded either radially, 

 tangentially or very irregularly, the gemmula; spicules, which re- 

 semble the parenchyma spicules in external form and variability as 

 to numbers. Ordinarily they are curved and thickly spined ; rarely 

 entirely smooth. 



, Remarks. Euspongilla laciistris is found in nearly all Europe, 

 as an inhabitant of both running and still water. As the above 

 Diagnosis shows, these fresh water sponges are liable to great varia- 

 tions, especially in external form, and in the quantity, as well as 

 the form, of the parenchyma and gemmulse-spicules. Based on 

 these variations, Euspongilla lacustris would be divided into many 

 species and varieties. Without doubt we have here a very variable 

 fundamental (grund) type, out of which new species are beginning 

 to form. Future careful researches, principally by experiment, 

 will show where lies tlie cause of the above mentioned variability in 

 the form and quantity of the parenchyma and gemmulaj spicules. 



Let us first examine those forms that have been looked upon as 

 indicating distinct species. 



We may take as typical, that form in which there are but few par- 

 enchyma spicules in the tissues, and in which the gemmulse are per- 

 fectly smooth and but sparingly supplied with covering spicules (Beleg- 

 nadeln). This form of Euspongilla lacustris has been regarded by 

 Lieberkuhn, and partly by Bowerbank, as the real Spongilla lacus- 

 tris ; and I have also considered it such, in my paper "Die Siiss- 

 wasser Schwiimme Bohmens" and so it is also represented by Retzer. 

 In my "Monograph" I have, however, at the same time, pointed out that 

 in one and the same colony (Stoecke), other forms are found with 

 rough parenchyma spicules and with covering spicules (Belegnadeln) 

 and seem to indicate a transitional stage towards those that have 

 been looked upon as distinct species. 



The nearest of these is Spoiigilla leiberkuhnii described by Noll 

 (Zoologischer Garten) in 1870, and also by Retzer, who, under the 

 same name, described it as follows: "Forms encrustations on wood 

 and stones, from which rise, frequently free, cylindrical processes as 

 long as one's finger. Skeleton spicules smooth, gradually pointed, 



