370 WILSON. [Vol. IX. 



takes place in the region of the cubical cells, and the larval 

 epithelium is not lost, but is modified cell by cell. 



The larva of Amorphina (Schmidt, 22) is a solid larva ciliated 

 all over. The cilia are lost at the posterior end. In the same 

 paper, Schmidt describes the larva of a species of Esperia. The 

 larva is solid and is ciliated all over. The cilia are lost at one 

 pole, the spicules collecting at this pole. The larva of Reniera 

 (Schmidt, /. r.) is a solid ciliated form with a deeply pigmented 

 pole. The cilia on the pigmented pole are lost. There are 

 other observations by Metschnikoff (11) and Carter (2) to the 

 effect that the ectoderm is absent at the posterior pole of the 

 larvae of silicious sponges. In these cases, as in the case of 

 the My.xilla larva described by Vosmaer, it remains doubtful 

 until the early development is known, whether the larva is 

 really an egg-larva. 



It will be seen that the larvae of the above-mentioned 

 silicious sponges agree in fundamental respects. They all 

 consist of two germ layers : an inner parenchymatous mass 

 (mes-entoderm) and an outer layer of columnar ciliated cells 

 (ectoderm). At one pole, usually the posterior, the ectoderm 

 is apparently absent, the appearance being probably due to the 

 fact that at this pole it is composed of flat unciliated cells. 

 Like the e.^% larvae, the gemmule larvae (of Esperella and 

 Tedania) consist of two germ layers, an inner parenchymatous 

 mass (mes-entoderm) and an outer layer of columnar ciliated 

 cells (ectoderm), the columnar ciliated cells giving place to flat 

 unciliated cells at the posterior pole which is thus differenti- 

 ated. It is plain that the two sorts of larvae agree in essential 

 structure. 



There are other silicious sponges, Spongilla (6, 14), the 

 tetractinellid form Plakina monolopha (26), and Tedanione (see 

 mitc, p. 345), in which the larva has not the peculiar differenti- 

 ation of one of the poles which is seen in the above-mentioned 

 forms. But this differentiation is so common that it may fairly 

 be considered as typical of a large though ill-defined group of 

 sponges. 



Cause of the Resetnblance. — Accepting as a fact the resem- 

 blance between the egg and gemmule larvae in the possession 



