NOTES AND MEMORANDA. 175 



free-swimming larva thus consists of tliree different germ-lamellre. 

 The ectoderm is formed of a series of flagellate cylinder-cells. The 

 mesoderm is a much thicker mass, consisting of rounded amoeboid 

 cells. The entoderm is formed of a single row of transparent flat 

 polygonal cells. At the posterior narrow pole of the larva an 

 accumulation of the mesoderm cells takes place at an early period, 

 and occupies nearly a third or a half of the length of the lai'va. 

 In the anterior clear part of the larva there is found a large 

 gastric cavity. The skeleton is confined to the posterior dark part 

 of the larva. On the external surface of the free-swimming larva 

 are seen a number of ectodermal processes, of different shapes and 

 sizes, which are of no morphological signification. Between the 

 ectoderm and mesoderm of the larva is seen a clear interval, into 

 which the processes of the mesoderm cells project in many places, 

 and which is to be regarded as the body-cavity. The posterior mass 

 of mesoderm grows forwards, as a result of which the stomach-cavity 

 becomes very much narrowed. The larva fixes itself by means of the 

 ectoderm cells of its posterior half, and soon loses its original form 

 and assumes a flat discoid shape. Transverse sections of the body of 

 the larva in such very early stages of the metamorphosis, prove that 

 the at first simple gastric cavity does not disappear, although it is 

 much altered by the great increase of the mesoderm, but passes imme- 

 diately into the entodermal cavity of the adult S'pongilla, Very soon 

 after the larva becomes fixed, a number of the so-called ciliated 

 chambers make their appearance simultaneously at several points in 

 the mesoderm ; their development depends upon out-pushings of the 

 entoderm. The histological differentiation of these ciliated chambers, 

 which at first are covered with flat cells, takes place somewhat later, 

 after the first central opening of the young Spongilla has been formed. 

 This first opening, which must be regarded as the oral aperture, is not 

 formed by invagination of the ectoderm ( Barrels), but by a breaking 

 through of the mesoderm and entoderm cells on the upper walls of the 

 stomach-cavity. The oral orifice of Spongilla differs from that of 

 other animals in that it does not open externally directly, but into a 

 special cavity, which is to be considered as the body-cavity. The 

 ectoderm and entoderm are always separate in Spongilla ; the margins 

 of the oral aperture do not become fused with the ectoderm. Soon 

 after the formation of the oral orifice, some of the so-called " ingestivo 

 aijertures" make their appearance. In the matter of development, 

 structure, and relation to other parts, these structures are perfectly 

 homologous with the oral aperture. 



The further development of the young Spongilla depends upon the 

 increase of the histological elements of the three membranes, in such 

 a way that each membrane gives rise only to elements of the same 

 morphological significance. The formation of the ciliated chambers 

 by division or by budding of old already-formed chambers, I have 

 never seen. The so-called osculum is homologous, as its development 

 shows, to the porus dermalis. It consists of two layers only (meso- 

 derm and ectoderm). The full-grown Spongilla is formed of three 

 different membranes, which originate directly from those of the same 



