42 M. E. Metsclmikoff on the 



of the formed larva, which has already been observed in the 

 same species by Lieberkiihn, and in the nearly allied Dunster- 

 villia corcyrensis (Sycandra Humboldtu } Hack.) by Oscar 

 Schmidt. In agreement with these naturalists, especially the 

 latter, I have found all normally developed swarming Sycon- 

 larvse divided into parts of nearly equal size, only one of 

 which appeared to be composed of vibratile cylindrical cells, 

 and the other of unciliated spherical cells (fig. 5). The 

 former portion constituted a sort of hemisphere containing in 

 its interior a central cavity of no great size, in the vicinity of 

 which a great number of very fine brown pigment-granules 

 were accumulated (fig. 5,y). 



If two or three specimens of sexually mature Syca are 

 kept only for a few days in small glass capsules, the larvae 

 swarm out in great quantities in order to become further de- 

 veloped, i. e. to attach themselves. To observe the subsequent 

 processes, all that is necessary is to put a few object-bearers at 

 the bottom of the glass capsules, so that they may serve as 

 points of adhesion for the larvae ; but even without this 

 precaution the adherent young sponges may be detected, as 

 they adhere to all objects existing in the vessels, even the 

 smallest. 



The first process of postembryonic development consists in 

 the complete disappearance of the central cavity, by which 

 the upper {i. e. the vibratile) half of the larval body is per- 

 ceptibly reduced in size (fig. 6)*. Then commences the fusion 

 of the spherical cells of the hinder part into a compact mass — ■ 

 only one row, of the spherical cells in immediate contact with 

 the vibratile epithelium, being an exception, as these still 

 retain their integrity for a considerable time (figs. 6, 8, 9, d). 

 The larvae often attach themselves even during this stage ; 

 but not unfrequently they continue for some time longer in 

 their swarming activity, but without being thereby hindered 

 in their development. As one of the most important processes 

 occurring in this, the formation of the calcareous spicula must 

 be specially noticed. The rather brownish, unciliated, compact 

 mass of the hinder portion is the place in which the skeletal 

 structures originate; to be convinced of this, one need only 

 glance at figs. 7 and 8. It must also be mentioned as worthy 

 of notice that at first only long rod-like spicula are formed ; 

 so that at this early stage our Sycon passes through a state 

 which is persistent in the genus Sycyssa, a fact which may be 

 of significance in phylogenetic considerations. 



The principal thing in the metamorphosis is that the un- 



* The above-mentioned brown mass of granules collected into a central 

 aggregation, as shown in figs. 6-8. 



